docs: richer project documentation

* Adds new "Contributing" guide explaining how to contribute in various
  ways.

* Adds new "Developing" guide intended for developers.  Enriches the
  instructions WRT installing, updating, or repopulating the site.

* Amends introductory "README" file accordingly.

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>