Use antora to create and provide a docs sitemap.xml

[mbroz2]

Antora can build a sitemap for docs, so we should do that and have two sitemaps; our current top-level one and the the antora provided one for docs. Note that in order for the sitemap to get generated, we need the site.url key to be set in the playbook.

https://docs.antora.org/antora/2.3/run-antora/

[steven1046]

I think there was a reason why we couldn't add the site url to the playbook. I am trying to remember.

[Chrispie]

@steven1046 Do you remembered why it was not possible? :)

[steven1046]

I think it might have been because Antora was providing its own 404 page when supplying the site.url.