DOC: use mpl-sphinx-theme for navbar, social, logo - #21429

(Note, probably not crucial for 3.5.0)

This works now - mol-sphinx-theme had a bug...

The only disadvatage I see with this concept is that we don't see how the resulting pages look like when changing the mpl-sphinx-theme. After updating the theme, all downstream sites have to be rebuilt as well.

Mpl-Sphinx-theme has a doc build. These docs are built at release time, and devdocs are built periodically. The other docs are built when changed, but we could also add a "daily" build to them all.

Owee, I'm MrMeeseeks, Look at me.

There seem to be a conflict, please backport manually. Here are approximate instructions:

1. Checkout backport branch and update it.

$ git checkout v3.5.x
$ git pull

2. Cherry pick the first parent branch of the this PR on top of the older branch:

$ git cherry-pick -m1 4c71a1f0a3f5beb498795ed9c6ea27b0be34db98

3. You will likely have some merge/cherry-pick conflict here, fix them and commit:

$ git commit -am 'Backport PR #21429: DOC: use mpl-sphinx-theme for navbar, social, logo'

4. Push to a named branch :

git push YOURFORK v3.5.x:auto-backport-of-pr-21429-on-v3.5.x

5. Create a PR against branch v3.5.x, I would have named this PR:

"Backport PR #21429 on branch v3.5.x (DOC: use mpl-sphinx-theme for navbar, social, logo)"

And apply the correct labels and milestones.

Congratulation you did some good work! Hopefully your backport PR will be tested by the continuous integration and merged soon!

Remember to remove Still Needs Manual Backport label once the PR gets merged.

If these instruction are inaccurate, feel free to suggest an improvement.