Docs: minor improvements of the github action

[tristanlatr]

• The template github action will now make pydoctor run on PRs as well.
  But still only push documentation when comitting to the main branch.
• Added the required permission to the action because github changed the default permission a while ago now.
• Bump version of actions-gh-pages from v3 to v4/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.79%. Comparing base (5eb7328) to head (89b4639).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #876   +/-   ##
=======================================
  Coverage   92.79%   92.79%           
=======================================
  Files          47       47           
  Lines        8468     8468           
  Branches     1550     1550           
=======================================
  Hits         7858     7858           
  Misses        350      350           
  Partials      260      260           

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