doc-rst: include index only in subproject AND html

The index should only be included if the build of the sub-folder is done
with the html-builder (HTML output).

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
diff --git a/Documentation/process/index.rst b/Documentation/process/index.rst
index 6ee8187..cddf580 100644
--- a/Documentation/process/index.rst
+++ b/Documentation/process/index.rst
@@ -30,3 +30,11 @@
    volatile-considered-harmful
 
    development-process
+
+
+.. only::  subproject and html
+
+   Indices
+   =======
+
+   * :ref:`genindex`