Add project: readthedocs

<!-- Please make sure that the project was not already added or suggested to this best-of list. You can ensure this by searching the projects.yaml, the README.md, and the issue list. --> **Project details:** - Project Name: - Github URL: https://github.com/mkdocs/theme-readthedocs - Category: Theming - License: same as Material - Package Managers: same as Material **Additional context:** I know that `readthedocs` is one of the two main MkDocs themes (the other being the Material theme). However, there are efforts to decouple the `readthedocs` code from the main MkDocs repo, where the code can be found at [https://github.com/mkdocs/theme-readthedocs](https://github.com/mkdocs/theme-readthedocs). This mean that the number of GitHub stars can be shown for this theme. As this is possible for the readthedocs theme now, I suggest to add it.