[SPARK-46825][DOCS] Build Spark only once when building docs

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

As [suggested here][1], this change improves the documentation build so that it builds Spark at most one time, regardless of what API docs are requested in the build.

[1]: #44791 (comment)

### Why are the changes needed?

There is no need to build Spark multiple times when generating docs. In particular, building Scala and Python docs, or Scala and SQL docs, causes Spark to be built twice.

Fixing this problem saves us a couple of minutes.

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

No.

### How was this patch tested?

I built the docs as follows on `master` as well as on this branch:

```sh
time SKIP_RDOC=1 SKIP_PYTHONDOC=1 bundle exec jekyll build
```

The time results before and after this change are as follows:

```
before
------
real    6m48.815s
user    23m17.943s
sys     1m29.578s

after
-----
real    4m10.672s
user    14m10.130s
sys     1m0.773s
```

That's a savings of about 2.5 minutes.

Additionally, I diffed the generated `_site/` dir across `master` and this branch and confirmed they are essentially identical except for some general SQL examples files.

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

No.

Closes #44865 from nchammas/SPARK-46825-jekyll-build-spark-once.

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

docs/_plugins/build_api_docs.rb

@@ -33,12 +33,27 @@ def print_header(text)
   puts banner_bar
 end
 
+def build_spark_if_necessary
+  if $spark_package_is_built
+    return
+  end
+
+  print_header "Building Spark."
+  cd(SPARK_PROJECT_ROOT)
+  command = "build/sbt -Phive -Pkinesis-asl clean package"
+  puts "Running '#{command}'; this may take a few minutes..."
+  system(command) || raise("Failed to build Spark")
+  $spark_package_is_built = true
+end
+
 def build_scala_and_java_docs
+  build_spark_if_necessary
+
   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..."
+  command = "build/sbt -Pkinesis-asl unidoc"
+  puts "Running '#{command}'..."
   system(command) || raise("Unidoc generation failed")
 
   puts "Moving back into docs dir."
@@ -124,19 +139,8 @@ def build_scala_and_java_docs
   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
+  build_spark_if_necessary
 
   print_header "Building Python API docs."
   cd("#{SPARK_PROJECT_ROOT}/python/docs")
@@ -168,9 +172,7 @@ def build_r_docs
 end
 
 def build_sql_docs
-  if !$spark_package_is_built
-    build_spark_package
-  end
+  build_spark_if_necessary
 
   print_header "Building SQL API docs."
   cd("#{SPARK_PROJECT_ROOT}/sql")