Add a custom sitemap for Read The Docs

[oraNod]

This PR adds a custom sitemap with urls for all subprojects in the Ansible namespace.

• Disable the nikola plugins for generating sitemaps and robots.txt files. We don't use those anyway.
• Add a custom ansible-sitemap.xml and robots.txt files.
• Copy the custom ansible-sitemap.xml and robots.txt files to Read The Docs.

You can't overwrite the sitemap that Read The Docs generates. However you can control the robots.txt file and point to the custom sitemap file. We can then add this custom sitemap via the Google Search Console.

Reminder that before we move the subdomain, we'll need to update the nikola configuration and sitemaps with the docs.ansible.com subdomain.

[felixfontein]

How is the sitemap generated?

Besides that, LGTM.

[oraNod]

How is the sitemap generated?

Besides that, LGTM.

Thanks for the review @felixfontein I've added a maintainers file with details about generating the sitemap. I'll add on to this maintainers file (and update the README) for other bits in a separate PR.