Allow for new GH Markdown "alert" extension

[cagix]

GH introduced "alerts" with distinctive styling, like

> [!NOTE]
> Foo bar, wuppie fluppie! 

(see https://github.blog/changelog/2023-12-14-new-markdown-extension-alerts-provide-distinctive-styling-for-significant-content/)

Let's stick with Pandocs divs in Markdown content and use filters for export:

::: note
Foo bar, wuppie fluppie! 
:::

• Export to GH Markdown using "distinctive alerts"
• Export to Hugo using notice shortcode
• Export to Beamer using beamercolorbox (also beameruserguide.pdf; or block, alertblock, examples - cf. https://www.overleaf.com/learn/latex/Beamer%23Creating_a_table_of_contents)

This should probably be in line with #180 ...

[cagix]

Support für GitHub Alerts (Quotes mit Kategorie, https://docs.github.com/en/get-started/writing-on-github/getting-started-with-writing-and-formatting-on-github/basic-writing-and-formatting-syntax#alerts): GitHub, Pandoc (In, Out: https://pandoc.org/MANUAL.html#extension-alerts), Relearn/Hugo