Guidance on rst vs md for documentation

[zklaus]

Checklist

• I added a descriptive title
• I searched open reports and couldn't find a duplicate

What happened?

By virtue of the MyST parser extension, already now markdown is supported as an alternative format for the documentation next to the traditional restructured text. At the moment, this is mostly used for deep dives and a few other documents.

It might be a good idea to clarify the use in the contribution guidelines, either by saying both are fine, or to move forward with only one in the future (as a side note, for documents, automatic conversion is available, so switching over entirely is also an option).

Both can equally be used for docstrings, iirc.

It is also possible to identify clear areas where one should be used over the other. For example, we may decide to write docstrings in rst, but narrative documentation in md.

Additional Context

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