Refine CONTRIBUTING.md

[koppor]

We vote for proposal A

Proposal A:

Change CONTRIBUTING.md:

# Contributing

1. I am a programmer: Please go to <https://devdocs.jabref.org/contributing>.
2. I am not a programmer: Please go to <https://docs.jabref.org/contributing>.

2. Refine docs/contributing.md

• Add TOC
• Add the "two reviewer" thing of the former CONTRIBUTING.md

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Proposal B:

1. Delete CONTRIBUTING.md
2. Refine docs/contributing.md

We accept that we are not in line with "good repository" guidelines.

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Proposal C:

1. Move docs/contributing.md to CONTRIBUTING.md
2. Refine CONTRIBUTING.md (add TOC, ...)
3. Change devdocs publishing process:
4. copy CONTRIBUTING.md to docs/contributing.md
5. Add nav: header to docs/contributing.md

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• We accept that the "Edit" button on the contributing guide does not work any more.
• We follow the GitHub approach to have CONTRIBUTING.md at the main folder
• We don't want to have two CONTRIBUING.md files