Update Windows CI setup and make the manuals job its own workflow

[wilfwilson]

• Make the "Build manual" job be in its own workflow file, for separate dispatching etc
• Reduce code duplication by merging the Windows job with the Ubuntu jobs: the necessary code is now almost identical
• The Windows job is now run on pull requests too, since it seems quick enough (10 minutes?)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.08%. Comparing base (c7a228d) to head (9753277).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #158      +/-   ##
==========================================
- Coverage   95.21%   95.08%   -0.13%     
==========================================
  Files          35       36       +1     
  Lines        2696     2729      +33     
  Branches      110      110              
==========================================
+ Hits         2567     2595      +28     
- Misses        109      114       +5     
  Partials       20       20              

see 1 file with indirect coverage changes

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