Fix empty page link in sysadm instance

Related to T3154

sysadm/support-services/index.rst

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

[Phabricator Migration user]

Some references in the commit message have been migrated:

• T3154 is now swh/infra/sysadm-environment#3154 (closed)