Add exclude + additionals option to AssembleDownstreamDocumentation

Why:

 * downstreamdoc.yaml is used to generate the list of docs used by
  downstream doc builds and today its hardcoded thus very limited
  for downstream docs ability to adjust without changing the quarkus
  repo.

This change addreses the need by:

 * adding a DOWNSTREAM_CONFIG_FILE to *optionally* override the
   hardcoded downstreamdoc.yaml
 * adding a DOWNSTREAM_ADDITONALS env var to the downstream doc build
   that can be used to add docs to the list of docs used by
   downstream doc builds
 * adding a DOWNSTREAM_EXCLUDES env var to the downstream doc build
   that can be used to exclude docs from the list of docs used by
   downstream doc builds + additionals

(cherry picked from commit 865a882)

docs/src/main/java/io/quarkus/docs/generation/AssembleDownstreamDocumentation.java

@@ -1,11 +1,17 @@
 package io.quarkus.docs.generation;
 
+//These are here to allow running the script directly from command line/IDE
+//The real deps and call are in the pom.xml
+//DEPS org.jboss.logging:jboss-logging:3.4.1.Final
+//DEPS com.fasterxml.jackson.core:jackson-databind:2.12.3
+//DEPS com.fasterxml.jackson.dataformat:jackson-dataformat-yaml:2.8.0.rc1
 import java.io.File;
 import java.io.IOException;
 import java.io.UncheckedIOException;
 import java.nio.file.Files;
 import java.nio.file.Path;
 import java.nio.file.StandardCopyOption;
+import java.util.Arrays;
 import java.util.Comparator;
 import java.util.List;
 import java.util.Map;
@@ -88,7 +94,30 @@ public static void main(String[] args) throws Exception {
             ObjectMapper yamlObjectMapper = new ObjectMapper(new YAMLFactory());
             yamlObjectMapper.disable(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES);
 
-            ConfigFile configFile = yamlObjectMapper.readValue(new File("downstreamdoc.yaml"), ConfigFile.class);
+            String configFilePath = System.getenv("DOWNSTREAM_CONFIG_FILE");
+            if (configFilePath == null) {
+                configFilePath = "downstreamdoc.yaml";
+            }
+            ConfigFile configFile = yamlObjectMapper.readValue(new File(configFilePath), ConfigFile.class);
+
+            String additionals = System.getenv("DOWNSTREAM_ADDITIONALS");
+            if (additionals != null) {
+                String[] additional_files = additionals.split(",");
+                LOG.info("Additional files: " + Arrays.toString(additional_files));
+                for (String file : additional_files) {
+                    configFile.guides.add(file);
+                }
+            }
+
+            String excludes = System.getenv("DOWNSTREAM_EXCLUDES");
+            if (excludes != null) {
+                String[] excludePatterns = excludes.split(",");
+                LOG.info("Excluding patterns: " + Arrays.toString(excludePatterns));
+                for (String pattern : excludePatterns) {
+                    Pattern regexPattern = Pattern.compile(pattern);
+                    configFile.guides.removeIf(guide -> regexPattern.matcher(guide).find());
+                }
+            }
 
             Set<Path> guides = new TreeSet<>();
             Set<Path> simpleIncludes = new TreeSet<>();