Updating docs and fixing API docs on readthedocs

[chummels]

Something changed somewhere about a year ago in a dependency and our API docs haven't been showing up on readthedocs (see Issue #156 ). This PR fixes most of that problem. It cannot display LightRay or the Ray generator functions yet, though, because right now stable yt_astro_analysis still uses an old broken reference in yt, and it's hosts the superclass for LightRay. But hopefully this will get addressed when a new version of yt_astro_analysis is released.

While I was at it, I updated the docs to also fix #152 and #173 and generally provide more instructions on how to do stuff.

[coveralls]

Pull Request Test Coverage Report for Build 4722ffc4-a474-42ab-b2ce-81adaf5f1b8a

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 75.619%

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Totals
Change from base Build 99405a3a-f1bf-452f-96ef-160be290ca1d:	0.0%
Covered Lines:	1740
Relevant Lines:	2301

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💛 - Coveralls