ENH: add the theme version in the footer

[12rambau]

Fix #990

I looked at what other themes were doing and as suggested in the issue, a lot of them are using

Created using Sphinx 5.3.2 and PyData Sphinx Theme 0.11.0.

To do that I should have overwritten the sphinx-version component entirely which is not the route taken by the theme where all the components displayed in the footer are independent. So I created a specific component that can be added to the footer.

it displays as such and can be set independently from the Sphinx version:

[choldgraf]

This is nice, thanks! A couple quick thoughts but it generally looks good

[choldgraf]

Could we put this in a little span element in case people want to style it?

[12rambau]

the whole thing is encapsulated in a p element with the theme-version class isn't it sufficient?

[choldgraf]

If you prefer not to have the span, it's fine w me. I tend to wrap everything that is "semantically different" from anything else in its own class but that is maybe overkill 😅

In the future if somebody really wants it they will ask for it and we can consider adding it in

[12rambau]

I don't have a strong opinion on this one, I simply copied the structure of the sphinx-version.html template.
If you want I can add a span to both or keep it as it is

[choldgraf]

Oops - noticed one more thing

[choldgraf]

I think we should just merge it in and iterate from there - thanks for this improvement!