Stand-alone documentation

[seifertm]

This PR creates a stand-alone HTML documentation using Sphinx and ReStructuredText.

The structure roughly follows the Diátaxis documentation framework in that it divides the content into four categories. Pytest-asyncio doesn't have tutorials and how-to guides at the moment, so these categories are missing. The Explanation category was named Concepts. The documentation contains a supplementary section about how users can get support for pytest-asyncio.

Most contents of the stand-alone documentation were extracted from the README. However, the contents have been restructured and rephrased to fit into the new structure.

The docs can be built by entering the docs folder and running make html.

@asvetlov @Tinche I would like to add you to the corresponding readthedocs project. Please give me your account names on that site so I can give you access to the RTD-project.

Closes #408

[codecov-commenter]

Codecov Report

Base: 92.41% // Head: 92.41% // No change to project coverage 👍

Coverage data is based on head (90bd462) compared to base (d39589c).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #448   +/-   ##
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  Coverage   92.41%   92.41%           
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  Files           3        3           
  Lines         277      277           
  Branches       38       38           
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  Hits          256      256           
  Misses         13       13           
  Partials        8        8           

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