[SPARK-46764][DOCS] Reorganize script to build API docs

### What changes were proposed in this pull request?

We are abusing Jekyll's plugin system to set flags for what API docs to build. This change maintains this overall status quo but makes the following improvements:

- Rename the plugin file to be in line with its actual purpose.
- Organize the code into functions so it's easier to follow and understand.
- Print section headers that are easier to find in the output when building the docs.

The behavior of the documentation build otherwise remains unchanged.

### Why are the changes needed?

This should make maintaining this part of the doc building workflow easier.

### Does this PR introduce _any_ user-facing change?

No.

### How was this patch tested?

In several ways:
- I built the docs with various skip flags set and confirmed the build succeeds. I also manually browsed through some of the Scala, Java, Python, and SQL API docs.
    - The only docs I didn't test building were the R docs, because I do not have R installed locally.
    - Here is the build output on my machine for `SKIP_RDOC=1`: [build-docs.log.zip][build-log]
    - There are some errors output when building the Java unidoc, but these are [present already in master][already].
- I built the docs against `master` and diffed the resulting `_site/` directory against the one output by this branch.
    - The `_site/` directories are identical except for minor differences in some general SQL function example files.
    - Here is the diff: [site.diff.zip][site-diff]
- I also confirmed that `./dev/change-scala-version.sh 2.13` runs successfully (though I didn't try to run the modified scripts after that).

[build-log]: https://github.com/apache/spark/files/13983386/build-docs.log.zip
[already]: https://github.com/apache/spark/actions/runs/7594517103/job/20686140435#step:30:3876
[site-diff]: https://github.com/apache/spark/files/14027280/site.diff.zip

### Was this patch authored or co-authored using generative AI tooling?

No.

Closes #44791 from nchammas/create-api-docs.

Authored-by: Nicholas Chammas <nicholas.chammas@gmail.com>
Signed-off-by: Hyukjin Kwon <gurwls223@apache.org>

dev/change-scala-version.sh

@@ -81,13 +81,13 @@ sed_i '1,/<scala\.binary\.version>[0-9]*\.[0-9]*</s/<scala\.binary\.version>[0-9
   "$BASEDIR/pom.xml"
 
 # Update source of scaladocs
-echo "$BASEDIR/docs/_plugins/copy_api_dirs.rb"
+echo "$BASEDIR/docs/_plugins/build_api_docs.rb"
 if [ $TO_VERSION = "2.13" ]; then
-  sed_i '/\-Pscala-'$TO_VERSION'/!s:build/sbt:build/sbt \-Pscala\-'$TO_VERSION':' "$BASEDIR/docs/_plugins/copy_api_dirs.rb"
+  sed_i '/\-Pscala-'$TO_VERSION'/!s:build/sbt:build/sbt \-Pscala\-'$TO_VERSION':' "$BASEDIR/docs/_plugins/build_api_docs.rb"
 else
-  sed_i 's:build/sbt \-Pscala\-'$FROM_VERSION':build/sbt:' "$BASEDIR/docs/_plugins/copy_api_dirs.rb"
+  sed_i 's:build/sbt \-Pscala\-'$FROM_VERSION':build/sbt:' "$BASEDIR/docs/_plugins/build_api_docs.rb"
 fi
-sed_i 's/scala\-'$FROM_VERSION'/scala\-'$TO_VERSION'/' "$BASEDIR/docs/_plugins/copy_api_dirs.rb"
+sed_i 's/scala\-'$FROM_VERSION'/scala\-'$TO_VERSION'/' "$BASEDIR/docs/_plugins/build_api_docs.rb"
 
 echo "$BASEDIR/dev/mima"
 if [ $TO_VERSION = "2.13" ]; then

docs/_plugins/build_api_docs.rb

@@ -0,0 +1,205 @@
+#
+# Licensed to the Apache Software Foundation (ASF) under one or more
+# contributor license agreements.  See the NOTICE file distributed with
+# this work for additional information regarding copyright ownership.
+# The ASF licenses this file to You under the Apache License, Version 2.0
+# (the "License"); you may not use this file except in compliance with
+# the License.  You may obtain a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+
+# This include enables functions like `cd` and `cp_r`.
+require 'fileutils'
+include FileUtils
+
+THIS_DIR = File.dirname(__FILE__)
+SPARK_PROJECT_ROOT = File.expand_path(THIS_DIR + "/../..")
+$spark_package_is_built = false
+
+def print_header(text)
+  banner = "* #{text} *"
+  banner_bar = "*" * banner.size
+
+  puts ""
+  puts banner_bar
+  puts banner
+  puts banner_bar
+end
+
+def build_scala_and_java_docs
+  print_header "Building Scala and Java API docs."
+  cd(SPARK_PROJECT_ROOT)
+
+  command = "build/sbt -Pkinesis-asl clean compile unidoc"
+  puts "Running '#{command}'; this may take a few minutes..."
+  system(command) || raise("Unidoc generation failed")
+
+  puts "Moving back into docs dir."
+  cd("docs")
+
+  puts "Removing old docs"
+  system("rm -rf api")
+
+  # Copy over the unified ScalaDoc for all projects to api/scala.
+  # This directory will be copied over to _site when `jekyll` command is run.
+  source = "../target/scala-2.13/unidoc"
+  dest = "api/scala"
+
+  puts "Making directory " + dest
+  mkdir_p dest
+
+  # From the rubydoc: cp_r('src', 'dest') makes src/dest, but this doesn't.
+  puts "cp -r " + source + "/. " + dest
+  cp_r(source + "/.", dest)
+
+  # Append custom JavaScript
+  js = File.readlines("./js/api-docs.js")
+  js_file = dest + "/lib/template.js"
+  File.open(js_file, 'a') { |f| f.write("\n" + js.join()) }
+
+  # Append custom CSS
+  css = File.readlines("./css/api-docs.css")
+  css_file = dest + "/lib/template.css"
+  File.open(css_file, 'a') { |f| f.write("\n" + css.join()) }
+
+  # Copy over the unified JavaDoc for all projects to api/java.
+  source = "../target/javaunidoc"
+  dest = "api/java"
+
+  puts "Making directory " + dest
+  mkdir_p dest
+
+  puts "cp -r " + source + "/. " + dest
+  cp_r(source + "/.", dest)
+
+  # Begin updating JavaDoc files for badge post-processing
+  puts "Updating JavaDoc files for badge post-processing"
+  js_script_start = '<script defer="defer" type="text/javascript" src="'
+  js_script_end = '.js"></script>'
+
+  javadoc_files = Dir["./" + dest + "/**/*.html"]
+  javadoc_files.each do |javadoc_file|
+    # Determine file depths to reference js files
+    slash_count = javadoc_file.count "/"
+    i = 3
+    path_to_js_file = ""
+    while (i < slash_count) do
+      path_to_js_file = path_to_js_file + "../"
+      i += 1
+    end
+
+    # Create script elements to reference js files
+    javadoc_jquery_script = js_script_start + path_to_js_file + "lib/jquery" + js_script_end;
+    javadoc_api_docs_script = js_script_start + path_to_js_file + "lib/api-javadocs" + js_script_end;
+    javadoc_script_elements = javadoc_jquery_script + javadoc_api_docs_script
+
+    # Add script elements to JavaDoc files
+    javadoc_file_content = File.open(javadoc_file, "r") { |f| f.read }
+    javadoc_file_content = javadoc_file_content.sub("</body>", javadoc_script_elements + "</body>")
+    File.open(javadoc_file, "w") { |f| f.puts(javadoc_file_content) }
+  end
+  # End updating JavaDoc files for badge post-processing
+
+  puts "Copying jquery.min.js from Scala API to Java API for page post-processing of badges"
+  jquery_src_file = "./api/scala/lib/jquery.min.js"
+  jquery_dest_file = "./api/java/lib/jquery.min.js"
+  mkdir_p("./api/java/lib")
+  cp(jquery_src_file, jquery_dest_file)
+
+  puts "Copying api_javadocs.js to Java API for page post-processing of badges"
+  api_javadocs_src_file = "./js/api-javadocs.js"
+  api_javadocs_dest_file = "./api/java/lib/api-javadocs.js"
+  cp(api_javadocs_src_file, api_javadocs_dest_file)
+
+  puts "Appending content of api-javadocs.css to JavaDoc stylesheet.css for badge styles"
+  css = File.readlines("./css/api-javadocs.css")
+  css_file = dest + "/stylesheet.css"
+  File.open(css_file, 'a') { |f| f.write("\n" + css.join()) }
+end
+
+def build_spark_package
+  print_header "Building Spark package."
+  cd(SPARK_PROJECT_ROOT)
+  command = "build/sbt clean package -Phive"
+  puts "Running '#{command}'; this may take a few minutes..."
+  system(command) || raise("Failed to build Spark")
+  $spark_package_is_built = true
+end
+
+def build_python_docs
+  if !$spark_package_is_built
+    build_spark_package
+  end
+
+  print_header "Building Python API docs."
+  cd("#{SPARK_PROJECT_ROOT}/python/docs")
+  system("make html") || raise("Python doc generation failed")
+
+  puts "Moving back into docs dir."
+  cd("../../docs")
+
+  puts "Making directory api/python"
+  mkdir_p "api/python"
+
+  puts "cp -r ../python/docs/build/html/. api/python"
+  cp_r("../python/docs/build/html/.", "api/python")
+end
+
+def build_r_docs
+  print_header "Building R API docs."
+  cd("#{SPARK_PROJECT_ROOT}/R")
+  system("./create-docs.sh") || raise("R doc generation failed")
+
+  puts "Moving back into docs dir."
+  cd("../docs")
+
+  puts "Making directory api/R"
+  mkdir_p "api/R"
+
+  puts "cp -r ../R/pkg/docs/. api/R"
+  cp_r("../R/pkg/docs/.", "api/R")
+end
+
+def build_sql_docs
+  if !$spark_package_is_built
+    build_spark_package
+  end
+
+  print_header "Building SQL API docs."
+  cd("#{SPARK_PROJECT_ROOT}/sql")
+  system("./create-docs.sh") || raise("SQL doc generation failed")
+
+  puts "Moving back into docs dir."
+  cd("../docs")
+
+  puts "Making directory api/sql"
+  mkdir_p "api/sql"
+
+  puts "cp -r ../sql/site/. api/sql"
+  cp_r("../sql/site/.", "api/sql")
+end
+
+if not (ENV['SKIP_API'] == '1')
+  if not (ENV['SKIP_SCALADOC'] == '1')
+    build_scala_and_java_docs
+  end
+
+  if not (ENV['SKIP_PYTHONDOC'] == '1')
+    build_python_docs
+  end
+
+  if not (ENV['SKIP_RDOC'] == '1')
+    build_r_docs
+  end
+
+  if not (ENV['SKIP_SQLDOC'] == '1')
+    build_sql_docs
+  end
+end