Add how to use apidoc guide/doc to our user docs

[agjohnson]

There are a number of different hacks for getting autoapi command to work on RTD, but there is now a sphinxcontrib extension that does it for users:
https://github.com/sphinx-contrib/apidoc

This would be good for us to highlight, so that users aren't trying to do this themselves or wondering why apidoc command can't execute.

A few things to touch on:

• Why apidoc command isn't used by RTD
• How to use the apidoc extension in a project

[stale]

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