Merge duplicate CONTRIBUTING contents

[tkoyama010]

Overview

We have deplicate contents in CONTRIBUTING.md and Contributing. It is not good that we have to write same thing in 2 places. I merge the content and save it to CONTRIBUTING.md CONTRIBUTING.rst.

Details

• None

resolve #2105

[adeak]

Thanks for tackling this @tkoyama010 (and for noticing the issue in the first place).

I think most contributors would be reading the online docs version, so we should definitely have an rst rendered there. The question is whether we just use a stub CONTRIBUTING.md (to be read on github or in a terminal) that points to the online docs, or whether to turn it into an .rst and keep the duplicate content (but in a way that doesn't need manual editing back and forth, i.e. by including the in-terminal version into the website version).

I can't tell which one is better, personally.

[tkoyama010]

Having seen the discussion at #2105, I think it is better to follow the authors.rst example as @banesullivan said.

This is exactly what we do in doc/getting-started/authors.rst: https://github.com/pyvista/pyvista/blame/main/doc/getting-started/authors.rst#L11

The rst file can be checked on GitHub without any problem, so I think it is fine to turn markdown into rst.

[akaszynski]

Thanks @tkoyama010! Agree with using rst as we can avoid duplication.