Documentation: Add missing footer that links to plugin index page

Change-Id: I125b821bf94bb1ecfaf2600b7aaf73448f06b1ed
diff --git a/src/main/resources/Documentation/build.md b/src/main/resources/Documentation/build.md
index 8232067..fae7b0c 100644
--- a/src/main/resources/Documentation/build.md
+++ b/src/main/resources/Documentation/build.md
@@ -70,3 +70,7 @@
 
 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