[DOCS] Enable sitemap creation for the Salt ref docs

[scriptautomate-bc]

We should be generating a sitemap listing for the Sphinx-generated website.
Once this is in place, a sitemap listing should also be created for the salt baseline repo itself that points to each of the sitemaps of:

• salt install guide
• salt user guide
• salt ref docs

[scriptautomate-bc]

Related:

• Enable sitemap creation for the user guide salt-user-guide#1
• Enable sitemap creation for the install guide salt-install-guide#30