Documentation: Add missing footer that links to plugin index page

Change-Id: Ia2b0995e491260ade8934fb6d1c1d188fc0988a5
diff --git a/src/main/resources/Documentation/build.md b/src/main/resources/Documentation/build.md
index b16bba2..6960506 100644
--- a/src/main/resources/Documentation/build.md
+++ b/src/main/resources/Documentation/build.md
@@ -68,3 +68,7 @@
 
 How to build the Gerrit Plugin API is described in the [Gerrit
 documentation](../../../Documentation/dev-bazel.html#_extension_and_plugin_api_jar_files).
+
+[Back to @PLUGIN@ documentation index][index]
+
+[index]: index.html