Remove jQuery from mkdocs theme

[squidfunk]

Since the mkdocs theme updated to Bootstrap 5, jQuery is not necessary anymore. I've replaced all uses of jQuery with common browser APIs and adapted the use of the modals to Bootstrap 5. My testing shows that master behaves exactly as if this PR were merged. If not, please report errors here and I'll fix them.

I don't think jQuery can be removed from the readthedocs theme.

Closes #3648

[squidfunk]

Side note: merging this PR will reduce the payload that the browser needs to download by 90kb (30kb gzipp'd), which is an improvement of -14% (currently 213kb gzipp'd). Thus, the MkDocs core theme should load faster!

[waylan]

Looks good to me. It is great to see this being worked on. I looked through the changes and they look good to me, but I'm weakest with JavaScript so I may have missed something.

[squidfunk]

Thanks for taking the time to look at this! ❤️

Do we want to release this with MkDocs 1.6.0, or afterwards? I've tested all that was apparent to me from reading the code and have found no problems (except for the visual regression mentioned in #3493 (comment), which are not related to the changes in this PR), but I'm also not a heavy user of the mkdocs theme.

If it should go into 1.6.0, we can merge it.

[waylan]

I would include this in 1.6.0 if we can.

[tomchristie]

I would include this in 1.6.0 if we can.

Seems reasonable. See also... #3648 (comment)

[squidfunk]

Great! Since everybody seems to agree that we want this in 1.6.0, let's merge it.