This PR locks in *almost* everything to switch over to [just-the-docs.com](https://just-the-docs.com)!

It:

- replaces all instances to the theme docs site (`https://just-the-docs.github.io/just-the-docs`) with `https://just-the-docs.com`
    - as @max06 has noted below, we do not change other links with the same `.github.io` prefix
- changes the `site.url` and `site.baseurl` in the theme docs' `_config.yml`
- adds a note in both the changelog and migration guide that a frozen version of `v0.3.3` docs is available at [https://v0-3-3-docs.just-the-docs.com/](https://v0-3-3-docs.just-the-docs.com/)

Closes #1160.