add GoatCounter analytics and Privacy Policy

[orbeckst]

• close #125
• see also https://github.com/MDAnalysis/MDAnalysis.github.io/issues/129 Privacy Policy

[codecov[bot]]

Codecov Report

Merging #127 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #127   +/-   ##
=======================================
  Coverage   97.93%   97.93%           
=======================================
  Files          12       12           
  Lines         775      775           
  Branches       91       91           
=======================================
  Hits          759      759           
  Misses         10       10           
  Partials        6        6           

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