[Documentation] Add information on building documentation and running tests locally

[elbeejay]

It'd be great if the documentation included "instructions for developers" on things like:

• Setting up a "developer" environment with an editable installation of the package
• Building the documentation locally
• Running the tests and calculating the code coverage locally

Current guidance on the contributing page does a good job covering how contributions should be made. It is unfortunate that the hyperlink in the readme (this) does not work, but since it works in the hosted documentation I'm wondering if changing it in the readme would break that link...

[g4brielvs]

@elbeejay Thanks! Excepts for tests (see #21), #53 should address these items.