chore: add publish-docs workflow

[aleclarson]

Summary

This PR has a 3 goals:

1. When a tag is pushed to radashi-org/radashi, publish a new version of the docs. One exception to this is when a beta version only contains patches that did not change anything in the docs/ folder.
2. Host multiple versions of the docs: one for the current stable version, one for the most recent beta version, one for the most recent alpha version (backed by the next branch, so breaking changes only), and finally the last two major versions before the current one. Old versions of the docs are phased out automatically. Only stable versions are published to the root directory. Others must have the "release ID" (e.g. v12.2-beta) in the URL.
3. Keep a versions.json file that includes all currently published versions of the docs, so we can provide a dropdown selector on the website.