Add instructions to contributing guide about executable docs

[maxrjones]

Motivated by the question in #3642 (comment)

TODO:

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/user-guide/*.md
• Changes documented as a new file in changes/
• GitHub Actions have all passed
• Test coverage is 100% (Codecov passes)

[d-v-b]

thanks max!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 60.90%. Comparing base (87d2041) to head (09d1041).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3645   +/-   ##
=======================================
  Coverage   60.90%   60.90%           
=======================================
  Files          86       86           
  Lines       10174    10174           
=======================================
  Hits         6196     6196           
  Misses       3978     3978           

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