DOC: Re-write the getting started section

[hamishwillee]

The getting started section of the docs has a number of problems:

• There is some duplication of commands for installing droneapi (ie done as a dependency and at the end)
• The instructions to install Mavproxy aren't really needed as this is done automatically as it is a Droneapi dependency in pip
• The instructions stop after installation. A get started should really go through to running an example.

[hamishwillee]

This was addressed in an earlier pull request