Support for multiple versions

[benbierens]

We'd like the docs site to allow users to switch between versions of codex easily.

[benbierens]

Findings so far:
All the existing links were using absolute paths. I had to rewrite them to relative paths for any versioning to work. Only image resources still use absolute paths because it is recommended to version them independently within the /public/ folder.

Started with vitepress-versioning-plugin: https://vvp.imb11.dev/guide/adding-version
Doesn't work out of the box. Couldn't get it to work. Hides too many of the details.

Used: https://github.com/IMB11/vitepress-versioning/tree/master/versions/0.1.0 as an example, but required modifications to get the sidebars to respect the selected version, and to make link-rewriting work correctly.

The current state of this branch works. I've added 0.0.1 and 0.1.0 as example versions, and added the version name to each sidebar entry and page title. A clever search-replace can easily remove this garbage later. But for now it's definitely needed.

All localization is gone. Adding localization in addition to versioning is likely to be an extra layer of cans of worms.