Refactor readthedocs-preview.yml into own workflow

[tpvasconcelos]

Ran into permission issues in #262 due to the use of the pull_request event instead of pull_request_target

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (e880c78) to head (a824ffc).
Report is 187 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #263   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           13        13           
  Lines          528       528           
  Branches        69        69           
=========================================
  Hits           528       528           

Flag	Coverage Δ
combined-src	99.24% <ø> (ø)

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