docs: fix the index to use a proper sphinx ref to the routing table

and prevent a sphinx warning.

This requires D728 to work properly.

Related to T1330

docs/index.rst

@@ -26,4 +26,6 @@ Reference Documentation
    :maxdepth: 2
 
    /apidoc/swh.storage
-   `URLs index <http-routingtable.html>`_
+
+
+* :ref:`routingtable`

[Phabricator Migration user]

Some references in the commit message have been migrated:

• D728 is now swh-docs!281 (closed)
• T1330 is now swh-docs#1330