Process admonitions with remark

[damianstasik]

Replaces #195

TODOs:

• Add support for GH admonitions
• Add support for the custom admonition syntax
• Handle malformed HTML

Checklist

• I have read the contribution guide.
• I have not used an AI coding assistant to create this PR.
• My contribution is compatible with the MPL-2.0 license and I have provided a DCO sign-off on all my commits.
• I have written all code in this PR myself OR I have marked all code I have not written myself (including modified code, e.g. copied from other places and then modified) with a comment indicating where it came from.

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