Prepare Contributor guides to be readable directly in GitHub

[Eric-Arellano]

Per #24 (comment), we are proposing for these 3 files to solely live as Markdown files simply live in the Qiskit repo, rather than being in the docs. That complies with normal open source expectations.

https://github.com/Qiskit/qiskit/blob/main/docs/maintainers_guide.rst
https://github.com/Qiskit/qiskit/blob/main/docs/deprecation_policy.rst
https://github.com/Qiskit/qiskit/blob/main/docs/contributing_to_qiskit.rst

Still, we need to convert them from RST to MD (not MDX) so that they render more usefully in GitHub rich preview. We can use Myst's converter: https://rst-to-myst.readthedocs.io/en/latest/cli.html#rst2myst-convert, but we need to check if there are any Sphinx-isms.

It's fine if we keep the files in RST, or we could also rewrite to Markdown. The important thing is that the rich preview in GitHub can render everything. Right now we have some Sphinx-isms like :ref:.

[Eric-Arellano]

@javabster fyi I updated this with the realization it doesn't matter if we rewrite to Markdown. It's fine to stick with RST. The important thing is removing Sphinx-isms like :ref: that don't render well in GitHub's rich preview.

[Eric-Arellano]

@javabster thanks for opening Qiskit/qiskit#11218 and Qiskit/qiskit#11217!!

Where will we point to these markdown guides from in the quantum.ibm.com docs, if it all? I think it'd probably be good to have a small Conributing part of our docs that simply links out to these new guides.