Documentation: Add missing footer that links to plugin index page

Change-Id: If5be574b2848ba308227e2019fcf2162693e36cc

src/main/resources/+Documentation/build.md

diff --git a/src/main/resources/+Documentation/build.md b/src/main/resources/+Documentation/build.md
index 8facc55..f4ca61c 100644
--- a/src/main/resources/+Documentation/build.md
+++ b/src/main/resources/+Documentation/build.md
@@ -79,3 +79,7 @@
 
 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
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