Generate man pages using tox & update them

• tox.ini: Add environment to update man pages

  Previously, one had to manually install click-man package, and remember to run the correct command. Now, we can simply run tox -e man to update the man pages.

• setup.py, CONTRIBUTING.rst: Recommend using tox to generate man pages

  Simplify our docs, by requesting users to run tox -e man to update man pages, instead of manually installing click-man and running the command manually.

• man: Refresh man pages

  Notable changes:

  • New source and artifact command groups
  • Man pages corresponding to obsolete commands, that were marked as hidden in Click, have now been removed.

Fixes #881 (closed). Fixes #880 (closed).