* 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.

Due to a wrong reST header, there were two top-level headers on that
page, leading to the subtitle “Style rules” being displayed on
https://pygments.org/docs/ as if it were a separate section.

Also harmonize the title.

Porting notes:

- tox handles Python environments automatically. Remove a bit of PYTHONPATH
  manipulation (that was using Python 2 code which always failed!)

- No `clean` target: `git clean -xdf` should fit the bill.

- No `reindent` target: the `reindent.py` script it was using does not
  exist (anymore?).

- No equivalent of tox-test-coverage, which was an artifact of the past,
  using nose.  Instead, the test-coverage target only is ported, which
  uses pytest, and works.