Port or remove footer.html template

[stsewd]

This template is used by our footer API

https://github.com/readthedocs/readthedocs.org/blob/main/readthedocs/api/v2/templates/restapi/footer.html

This template doesn't really make sense to be ported, so I guess we want to remove it? But if we do, that will break our old footer_html API, which may be used for old projects for some custom stuff.

ref https://github.com/readthedocs/readthedocs-corporate/pull/1807#discussion_r1729403446

[humitos]

I think it's fine deprecating this endpoint and not porting the template. Nobody will be using it after October 7th when addons became the default.