Improve doc/source

Here we enhance the rst docs (used for readthedocs):
* Simplify its index structure by reducing it to only 5 points:
  1. Deploy (engines & server providers)
  2. Verify
  3. Benchmark (scenarios, engine, contexts, plugins)
  4. Command Line Interface
  5. Implementation (all source docs moved here)
* Extend the 'Benchmark' page with info from the "Main concepts" page from Wiki
* Add some introductory text on Deploy & Verify parts based on that from Wiki
* Simplify headings by removing "The" and "Layer"

Change-Id: I04bb553aba04009634272a88a757baa8f2c8c359
Merge "Improve doc/source"