add support for markdown alerts

[bjudkewitz]

Problem Description

Github supports markdown alerts, which are very helpful for readmes and other documentation.

Markdown as such:

> [!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.

...renders like this on Github:

See also: https://docs.github.com/en/get-started/writing-on-github/getting-started-with-writing-and-formatting-on-github/basic-writing-and-formatting-syntax#alerts

Pdoc doesn't yet support it, which makes it hard to include files such as readme.md in pdoc documentation.

Proposal

Support markdown alerts.