Prepare this repository for Bootstrap v6

[julien-deramond]

Bootstrap is developing v6 on v6-dev. Once that reaches alpha, main on twbs/bootstrap becomes v6 and today's v5 moves to v5-dev there. We should set this repo up the same way ahead of time.

•  Create a v6-dev branch.
  • Swap the dependency, reinstall, regenerate the lockfile. Use the github repo reference as long as the v6 alpha is not released.
  • Change the docs links to https://v6-dev--twbs-bootstrap.netlify.app/
  • Change the README content
  • Change the DOM and classes to reflect Bootstrap v6 usage
  • ⚠️ Dependabot won't be able to detect changes — it can't bump a branch-pinned git dependency anyway. But we'll backport changes from main to v6-dev temporarily.

When v6-dev becomes main and main becomes v5-dev in Bootstrap repository

• Move main to v5-dev in this repository
• Move v6-dev to main in this repository
• Update the docs link
• Update the Bootstrap npm dependency to released alpha version