Devolve Contributing.md (#2352)

* Devolve Contributing.md

Move the content to the docs and website so it is displayed on
pygments.org, to make it easier to find.

- Regex dos and don'ts go to lexerdevelopment.rst

- The rest goes to a new file contributing.rst

- That makes the explanation of how lexers are tested in
  lexerdevelopment.rst redundant, so remove it. The bit on how to add a
  lexer goes to contributing.rst.

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