Mention "Hosted on Read the Docs" in the footer

[humitos]

Closes #1587

[agjohnson]

Not sure if I understand the need for this link from the underlying issue, but it feels a bit redundant or maybe even spammy. Is there a reason we need a second link to us?

[humitos]

We don't need a second link. I removed the old one in 14d07f9

[agjohnson]

To clarify, it's not just the link, it's the text that feels redundant/spammy. This will still render as:

Built with Sphinx using a theme provided by Read the Docs. Hosted by Read the Docs.

This still feels redundant, maybe spammy -- it doesn't feel to me like we need our name in there twice on every page.

Built with Sphinx and hosted on Read the Docs.

That seems fine.

I'm also fine not mentioning hosting. I think it's a surprise to users when there is conditional content like this -- locally this string isn't there but our builds produce different output.

[agjohnson]

This should probably be wrapped by show_sphinx or a specific configuration option. If the user selected show_sphinx = False, they will probably not like that we injected different content in the same space.