Add links to the GitHub repository from docs

[DominikPeters]

Currently it is difficult to get from the documentation to the GitHub repository, so I added a link to https://github.com/martinlackner/abcvoting/ from the first word on the home page, and adding a link to the bottom of the table of contents.

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.95%. Comparing base (321240e) to head (c5303b9).

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Additional details and impacted files

@@           Coverage Diff           @@
##           master      #87   +/-   ##
=======================================
  Coverage   91.95%   91.95%           
=======================================
  Files          11       11           
  Lines        3159     3159           
  Branches     1003     1000    -3     
=======================================
  Hits         2905     2905           
  Misses        146      146           
  Partials      108      108           

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