Add missing styles for Github's markdown alerts

[Steve-Tech]

Hi, the styles for !TIP, !IMPORTANT, and !CAUTION markdown alerts seemed to be missing, but pdoc otherwise recognises them fine. This PR adds those styles and accompanying svg icons.

For example, the alerts look like this now:

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For reference, here's Github's:

Note

Useful information that users should know, even when skimming content.

Tip

Helpful advice for doing things better or more easily.

Important

Key information users need to know to achieve their goal.

Warning

Urgent info that needs immediate user attention to avoid problems.

Caution

Advises about risks or negative outcomes of certain actions.

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Thanks,
Steve

[mhils]

Thanks, this is great! Would you mind adding a CHANGELOG entry please?

[Steve-Tech]

@mhils No worries! I've rebased and added a CHANGELOG entry.

[mhils]

Thanks! 🍰