handle multiple versions of the website on GitHub pages

This is a proposal on how to handle multiple versions of the website using GitHub pages. It could be a discussion, but they are not enabled on this repo 🙂

## One branch per version

The main branch is for documentation/website of the main branch of Bevyengine. Each version get its own branch, so one for 0.7, one for 0.8, ...
When creating a new branch from main, care should be taken to update the version used to refer to Bevy in text, in links, in actions, ...

## Branches are mirrored to other repositories

For all branches except main, they are mirrored to another repository on GitHub. There is a setup to do as part of each release: create a new repo, and setup the GitHub action to mirror any push to branches to the corresponding repository.
Those repository should not have any issue / PRs / ... They should just deploy their content to GitHub pages.
This allows us to work around GitHub limits on pages: one domain per repo, and size limit of pages

## Dropdown on the website

So that visitors can select the version they are using.
main branch is deployed as version unstable and can now be updated with Bevy instead of waiting for the release for a massive update

Review the current GitHub Pages deployment, branch structure, and GitHub Actions setup before choosing an implementation. Done means versioned website content can be deployed for each Bevy version, mirrored as proposed, and selected through a website version dropdown.

Written by the indexing model from the issue text.