We are essentially writing GFM e.g. inline HTML comments and GFM alerts.
So tell pandoc to parse/render as such. This resolves issues such as:
`> [!TIP]` being escaped to `> \[!TIP\]`.
The README/CONTRIBUTING files are authored with GitHub in mind, but we
want to re-use them for the docs website.
Replace the existing simple substitution with a pandoc AST-based filter.