Enable and use GitHub Alert syntax for admonitions

[llucax]

This allows admonitions/callouts/alerts to be rendered properly both in GitHub and mkdocs. On the downside, GitHub syntax is less flexible than mkdocs, so we can't have the admonitions be rendered inline end any more.

Because of this we only use the GitHub syntax for the README, as it is more important that the README is rendered correctly in GitHub than having it look a bit nicer in mkdocs.

[llucax]

Before:

After:

[shsms]

I wonder what requirement is missing.

[llucax]

I guess it got borked when I switched the merge queue from a "branch protection" to a "ruleset" #GitHub :unicorn: