Update OG-USA documentation

[rickecon]

This PR implements the following updates to the documentation.

• Update the copyright year in _config.yml
• Differentiate directories from the package with slashes (public_api.rst, contributor_guide.md)
• Update the BibTeX reference style.
• Basic stylistic and typographical changes.

cc: @jdebacker

[codecov-commenter]

Codecov Report

Merging #64 (8b09a3a) into master (da8dc39) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #64   +/-   ##
=======================================
  Coverage   78.35%   78.35%           
=======================================
  Files          19       19           
  Lines        1303     1303           
=======================================
  Hits         1021     1021           
  Misses        282      282           

Flag	Coverage Δ
unittests	78.35% <ø> (ø)

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[jdebacker]

Thanks @rickecon!