This PR consists of:

1. Fix broken links: alerts by mdbook-lintcheck
2. Fix Markdown files
3. Prettify css, js, md, and yaml file.
4. Add prettifier to the pre-commit hooks
5. Add mdbook-lintcheck to the CI
6. website-dev target in the Makefile.
7. Oh, and... new colour themes.


![image](https://user-images.githubusercontent.com/1428088/224781222-bd2a08f4-32d3-4acd-8a5b-45f2bd81f4bb.png)

Tested locally.


Please, somebody else, update the nix file to support the tooling
changes.