* add Terminal Tips Documentation - Issue #2131

* add newlines after ::, typo

* Add highlight directive

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Co-authored-by: Jean Abou Samra <jean@abou-samra.fr>

* 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.