Update new docs version picker to work on home and examples pages

[mdo]

Alternate fix to #36221.

This doesn't address every issue, but it fixes the broken links for Home and Examples. This also fixes the 404 for the utilities/opacity/ page. What remains is figuring out what to do about two situations:

• Disabled links for components added in later releases (e.g., https://twbs-bootstrap.netlify.app/docs/5.1/components/placeholders/)
• Providing a general "v5.1 docs link" instead of a page-specific link

TBD on both those.

/cc @julien-deramond

[mdo]

For more context on why we would keep this version picker, see #31980. We've had a handful of issues and more requests to better support jumping between versions of specific pages.

Across major versions, we need better linking and some updates for the previous major versions.

[julien-deramond]

I'm closing #36221 based on the content of this PR that fixes the blocking issues. As said in the description what remains is figuring out what to do about two situations.

Let's keep in mind that it would be safer in the future to wait for the beginning of a major version to change the sitemap of the documentation (e.g. moving or renaming some sections). Otherwise, between minor versions, if the sitemap of the documentation changes too much, it will result in a lot of 404s.

Don't know if it would be feasible but while I was testing this PR, one possible evolution would be to keep the anchor links as well:

• http://localhost:9001/docs/5.1/components/dropdowns/#dropup would redirect to https://getbootstrap.com/docs/5.0/components/dropdowns/#dropup
• http://localhost:9001/docs/5.1/components/dropdowns/#dropup-centered would redirect to https://getbootstrap.com/docs/5.0/components/dropdowns/#dropup-centered and since it doesn't exist, it will still display the top of the page.

[mdo]

Let's keep in mind that it would be safer in the future to wait for the beginning of a major version to change the sitemap of the documentation (e.g. moving or renaming some sections). Otherwise, between minor versions, if the sitemap of the documentation changes too much, it will result in a lot of 404s.

Definitely. No major restructuring of pages here. Only thing we should have to worry about is new pages, which we've accounted for with the added front matter.

Don't know if it would be feasible but while I was testing this PR, one possible evolution would be to keep the anchor links as well:

I'm going to skip that for now, definitely a nice to have. Also a little tougher since the headings can change between versions (e.g., ## Sass became ## CSS on a ton of pages). Not a huge deal because those links would still function, they just wouldn't scroll to the section.