Documentation: Add missing footer that links to plugin index page

Change-Id: I2f6d3aa2fcf369438b1c02d20bfaf4d7e0bce94d

src/main/resources/Documentation/build.md

diff --git a/src/main/resources/Documentation/build.md b/src/main/resources/Documentation/build.md
index 6382d72..e59c128 100644
--- a/src/main/resources/Documentation/build.md
+++ b/src/main/resources/Documentation/build.md
@@ -73,3 +73,7 @@
 
 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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