Misc documentation cleanup

[vytas7]

Documentation improvements and tweaks that are not highest prio, but would be nice to iron out:

• https://falcon.readthedocs.io/en/latest/community/contributing.html#reviews ⬅️ checkboxes within use the GitHub flavour of Markdown, which does not render exactly the same in the MyST Markdown flavour that Sphinx uses. Check if there is a suitable plugin, or, in the worst case, just convert CONTRIBUTING.md to ordinary bullet lists.