Cleanup a little of documentation_type from footer

[stsewd]

This doesn't remove documentation_type completely from
the footer, but makes it more clear what we are really using.

We don't use readthedocssinglehtml as proven in readthedocs/readthedocs-sphinx-ext#58
We don't set page for mkdocs projects: https://github.com/rtfd/readthedocs.org/blob/f1c15d4f22af0ba7ebf762df1d49f7c28e8d01e5/readthedocs/doc_builder/backends/mkdocs.py#L207-L207

Ref #4638

[humitos]

I don't have enough context to review this PR.

From what I can read, we are defining path to ends with / for sphinx_htmldir and .html otherwise. Why do you mention page in the description if the logic we are changing here is around path instead?

[stsewd]

Basically we have this

https://github.com/rtfd/readthedocs.org/blob/2f6fe90b1d3b78d7eec857ff796ed1a867b27457/readthedocs/restapi/views/footer_views.py#L97

And page slug comes from the code linked in the PR comment

https://github.com/rtfd/readthedocs.org/blob/2f6fe90b1d3b78d7eec857ff796ed1a867b27457/readthedocs/restapi/views/footer_views.py#L74

Which is None for both, singlehtm and mkdocs

[ericholscher]

Simple enough. 👍