Add exclude + additionals option to AssembleDownstreamDocumentation

Why:

 * downstreamdoc.yaml is used for two things:
    * to generate the list of docs used by downstream doc builds
    * used to match in github notifications to determine if a PR is
      a downstream relevant doc PR

This change addreses the need by:

 * 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

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

@@ -1,11 +1,18 @@
 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;
@@ -90,6 +97,26 @@ public static void main(String[] args) throws Exception {
 
             ConfigFile configFile = yamlObjectMapper.readValue(new File("downstreamdoc.yaml"), 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<>();
             Set<Path> includes = new TreeSet<>();