Clarify and fix the dev-quickstart doc some more

Clarify and fix up the dev-quickstart doc some more
based on user feedback.

Testing examples now use tox rather than invoking the
testr command directly.

Simplified the section on building the documentation by
removing the need to install a web server.

Co-Authored-By: David Shrewsbury <shrewsbury.dave@gmail.com>

Change-Id: Ibdff4b44a26f11a5a0e92efb4c451105f6ab3245
Merge "Clarify and fix the dev-quickstart doc some more"