build: Fix indexing so it works on HTML and PDF.

- Remove all genindex.rst files, these are generated and should not
  exist in our source.
- Fix the HTML templates so the local specialisation works.
- Add a index link to the sidebar for HTML.

Note, there is no TOC entry for the index in the PDF output and I
cannot figure out how to add one.

Closes #3503

develenv/genindex.rst

@@ -1,4 +0,0 @@
-.. comment SPDX-License-Identifier: CC-BY-SA-4.0
-
-Index
-=====

develenv/index.rst

@@ -47,4 +47,4 @@ to the Community Project hosted at http://www.rtems.org/.
 	utilities
 	command
 
-        genindex
+* :ref:`genindex`