Set up basic documentation

[leburgel]

While the README has had a very helpful update recently, I think it would be good to additionally have some basic library documentation which at least collects all the docstrings. In particular, this would make it possible to directly refer to OptimKit.jl methods using DocumenterInterLinks.jl, which would be very helpful in MPSKit.jl and PEPSKit.jl.

I checked the build locally, but the github pages will have to be set up after merging to actually get things published.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.31%. Comparing base (82b4306) to head (1c8aba0).

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #45   +/-   ##
=======================================
  Coverage   82.31%   82.31%           
=======================================
  Files           8        8           
  Lines         752      752           
=======================================
  Hits          619      619           
  Misses        133      133           

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