docs: Include module indices only when building standalone package doc

In order to remove warnings about /apidoc/*.rst files being included
multiple times in toc when building full swh documentation, prefer to
include module indices only when building standalone package documentation.

Related to T4496

docs/index.rst

@@ -7,9 +7,11 @@
    :caption: Contents:
 
 
-Indices and tables
-------------------
+.. only:: standalone_package_doc
 
-* :ref:`genindex`
-* :ref:`modindex`
-* :ref:`search`
+   Indices and tables
+   ------------------
+
+   * :ref:`genindex`
+   * :ref:`modindex`
+   * :ref:`search`

[Phabricator Migration user]

Some references in the commit message have been migrated:

• T4496 is now swh-docs#4496 (closed)