Update canonical site domain to be just-the-docs.com (#1211)

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.

README.md

@@ -5,7 +5,7 @@
 <p align="center">
     <h1 align="center">Just the Docs</h1>
     <p align="center">A modern, highly customizable, and responsive Jekyll theme for documentation with built-in search.<br>Easily hosted on GitHub Pages with few dependencies.</p>
-    <p align="center"><strong><a href="https://just-the-docs.github.io/just-the-docs/">See it in action!</a></strong></p>
+    <p align="center"><strong><a href="https://just-the-docs.com/">See it in action!</a></strong></p>
     <br><br><br>
 </p>
 
@@ -102,7 +102,7 @@ The theme is available as open source under the terms of the [MIT License](http:
 
 [Jekyll]: https://jekyllrb.com
 [Just the Docs Template]: https://just-the-docs.github.io/just-the-docs-template/
-[Just the Docs]: https://just-the-docs.github.io/just-the-docs/
+[Just the Docs]: https://just-the-docs.com
 [Just the Docs repo]: https://github.com/just-the-docs/just-the-docs
 [GitHub Pages]: https://pages.github.com/
 [Template README]: https://github.com/just-the-docs/just-the-docs-template/blob/main/README.md