Add documentation via Documenter.jl

[sloede]

This partially addresses #27.

After having to navigate to the sources of Preferences.jl for the umpteenth time because I forgot the details of the non-macro API, I decided to add Documenter.jl-based documentation. In addition, I added some small bits and pieces to the docs (such as an author and license section).

@staticfloat It would be great if you could set up the DOCUMENTER_KEY secret as described in the Documenter.jl docs & review this PR at your convenience. If you want only the bare minimum docs and not the additional changes I made, I can also revert some of them.

[codecov]

Codecov Report

Merging #56 (e914ecb) into master (e8082bc) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #56   +/-   ##
=======================================
  Coverage   88.05%   88.05%           
=======================================
  Files           2        2           
  Lines         134      134           
=======================================
  Hits          118      118           
  Misses         16       16           

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[sloede]

I changed the job name to build-docs since sometimes GitHub only refers to the job names without the workflow name, and in that case having only build is just very unhelpful.

[staticfloat]

Awesome! Let's see if it works!