UI: Simplifying "For Education"

* (addresses #218)
UI: Simplifying "For Education" - new intro tab

* adds new “Introduction” page to “Education” (addresses #234)
UI: URL changes  -> /getting-started/<experiment>

* (addresses #223)
UI: FIXED broken VM link

* (closes #235)
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>