Upgrade Bootstrap to version 5

[lb-]

Currently the Theme is using Bootstrap 4 and it would be good to upgrade to version 5.

Split from #252 / #250

Bootstrap 5 did remove jquery as a dependency. Not sure how broken that would be to upgrade, or if it would be a mostly drop-in replacement given our minimal bootstrap usage. https://getbootstrap.com/docs/5.2/migration/
Originally posted by @vsalvino in #252 (comment)

It may be worth considering a small clean up of unused styles in the process. See #64

[dassudip2001]

it used Bootstrap CDN ??

[lb-]

It's a front-end dependency see https://github.com/wagtail/sphinx_wagtail_theme/blob/main/package.json#L23

[dassudip2001]

i can fix it ? can i send a PR

[lb-]

Just a thought. Is it worth looking at something more lightweight instead of Bootstrap.

Maybe too lightweight - https://open-props.style/ - but super flexible with CSS Custom Properties as the core method of designs.

Not sure on other options, maybe we need to review the actual Bootstrap parts used.

Also need to consider the ability to customise and override things for others.