Add automatic conversion for CONTRIBUTING.rst

[abravalheri]

Related to #18. This would be more of a RFC than a PR...

A small drawback is that rst_to_myst requires Python >=3.7 (previously pyscaffoldext-markdown was compatible with Python 3.6).

I did some tests in my machine and the conversion works mostly fine. There is a bunch of {eval-rst} blocks throughout the document (specially for the .. todo directives).

The main problem is that the original template in PyScaffold use rst substitutions, and MyST cannot work very well with those even when the relevant extension is activated (the automatic conversion rely on 1:1 compatibility between these 2 syntaxes, but when running tox -e docs I can see some errors and the final HTML output does not seem to perform the correct substitution).

[FlorianWilhelm]

Thanks @abravalheri. So just to make sure I got it. The big plus of this solution is that we don't need to maintain the template docs in rst (PyScaffold) and md (this extension) but the extension just takes the original files from PyScaffold and translates them for us? If this works reliably I think that's a great idea.

If it does not, just adding a custom CONTRIBUTING.md would also be fine since it shouldn't change that often, right?

[abravalheri]

Hi @FlorianWilhelm, that is the gist of it.

Unfortunately the translation is not 100% yet. Although super close, I found an issue with replacements.

I think I will open an issue with rst-to-myst to see if there is something that can be done in that respect.

Please also notice that we have to drop support for Python 3.6 on pyscaffoldext-markdown (rst-to-myst requirement).