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

@@ -14,3 +14,12 @@
    graph/index
    export
    generate_subdataset
+
+.. only:: package_doc
+
+   Indices and tables
+   ------------------
+
+   * :ref:`genindex`
+   * :ref:`modindex`
+   * :ref:`search`
\ No newline at end of file