feat: Add basic styling to Alerts in the HTML exporter

[nathanlesage]

This PR contains the style changes I pulled out of #11836. See that PR for context.

EDIT: I just saw that this thing also includes the two changes from the other branch because I drafted this branch from the other one. I suspect we can first merge #11836 and then I can update this branch from main and get rid of the changes to the Markdown parser.

[nathanlesage]

It seems to have worked — the only changes currently showing in the diff are the three HTML files. Sorry again, and thanks for your help @jgm.

[jgm]

If you rebase against main and force push, you'll get a clean commit with just the style changes.

[nathanlesage]

So, that took a while, which is quite embarassing, but at least now I know what to do in such cases in the future… the branch is now clean and ready for review. Sorry about the hiccups.

[jgm]

Can you paste a screenshot that shows how these look with this CSS?

Also, if we do this, it might make sense to gate the inclusion of the CSS for alerts under a template variable which would be set automatically by the writer if there are actually alerts present. This keeps the size down for other cases.

My main reservation so far is that we don't do automatic styling of these in other formats; our thought was that there are too many ways to do it, so we wanted to leave it to users to handle with filters etc. However, in HTML everything can be done with CSS, so perhaps this is an exception.