Rewrite footnotes for downstream documentation

The modern version of footnotes is not supported in the downstream
tooling and we can't use the older version in our doc as it triggers
warnings.
Thus we rewrite the new format to the old format specifically for
downstream doc.

(cherry picked from commit 82f7b20)

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

@@ -61,6 +61,7 @@ public class AssembleDownstreamDocumentation {
             Pattern.CASE_INSENSITIVE + Pattern.MULTILINE);
     private static final String SOURCE_BLOCK_PREFIX = "[source";
     private static final String SOURCE_BLOCK_DELIMITER = "--";
+    private static final Pattern FOOTNOTE_PATTERN = Pattern.compile("footnote:([a-z0-9_-]+)\\[(\\])?");
 
     private static final String PROJECT_NAME_ATTRIBUTE = "{project-name}";
     private static final String RED_HAT_BUILD_OF_QUARKUS = "Red Hat build of Quarkus";
@@ -386,7 +387,7 @@ private static void copyAsciidoc(Path sourceFile, Path targetFile, Set<String> d
 
                 if (currentBuffer.length() > 0) {
                     rewrittenGuide.append(
-                            rewriteLinks(sourceFile.getFileName().toString(), currentBuffer.toString(), downstreamGuides,
+                            rewriteContent(sourceFile.getFileName().toString(), currentBuffer.toString(), downstreamGuides,
                                     titlesByReference, linkRewritingErrors));
                     currentBuffer.setLength(0);
                 }
@@ -399,7 +400,7 @@ private static void copyAsciidoc(Path sourceFile, Path targetFile, Set<String> d
 
         if (currentBuffer.length() > 0) {
             rewrittenGuide.append(
-                    rewriteLinks(sourceFile.getFileName().toString(), currentBuffer.toString(), downstreamGuides,
+                    rewriteContent(sourceFile.getFileName().toString(), currentBuffer.toString(), downstreamGuides,
                             titlesByReference, linkRewritingErrors));
         }
 
@@ -413,7 +414,7 @@ private static void copyAsciidoc(Path sourceFile, Path targetFile, Set<String> d
         Files.writeString(targetFile, rewrittenGuideWithoutTabs.trim());
     }
 
-    private static String rewriteLinks(String fileName,
+    private static String rewriteContent(String fileName,
             String content,
             Set<String> downstreamGuides,
             Map<String, String> titlesByReference,
@@ -454,6 +455,14 @@ private static String rewriteLinks(String fileName,
             return "[[" + mr.group(1) + "]]";
         });
 
+        content = FOOTNOTE_PATTERN.matcher(content).replaceAll(mr -> {
+            if (mr.group(2) != null) {
+                return "footnoteref:[" + mr.group(1) + "]";
+            }
+
+            return "footnoteref:[" + mr.group(1) + ", ";
+        });
+
         return content;
     }