sysadm: Fix toctree contains reference to nonexisting document

Related to T3648

sysadm/support-services/index.rst

+.. _empty_page:
+
+Empty page
+==========
+
+.. todo::
+   This page is a work in progress.

[Phabricator Migration user]

Some references in the commit message have been migrated:

• T3648 is now #3648 (closed)