Add project API version on each HTML page.

[tristanlatr]

Fixes #333

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.33%. Comparing base (cac4486) to head (fda3193).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #936   +/-   ##
=======================================
  Coverage   93.33%   93.33%           
=======================================
  Files          48       48           
  Lines        8912     8912           
  Branches     1636     1636           
=======================================
  Hits         8318     8318           
  Misses        337      337           
  Partials      257      257           

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[github-actions]

According to pydoctor_primer, this change doesn't affect pydoctor warnings on a corpus of open source code. ✅