[DOP-14061] Replace asserts in documentation with doctest style

[dolfinus]

Change Summary

Instead of syntax:

assert some == "other"

Use doctest style:

>>> some
"other"

This should make examples easier to understand. See https://onetl--273.org.readthedocs.build/en/273/.

Also run pytest on these docstrings, to increase coverage.

Related issue number

Checklist

• Commit message and PR title is comprehensive
• Keep the change as small as possible
• Unit and integration tests for the changes exist
• Tests pass on CI and coverage does not decrease
• Documentation reflects the changes where applicable
• docs/changelog/next_release/<pull request or issue id>.<change type>.rst file added describing change
  (see CONTRIBUTING.rst for details.)
• My PR is ready to review.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.68%. Comparing base (24660e5) to head (de20bd5).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #273      +/-   ##
===========================================
+ Coverage    94.46%   94.68%   +0.21%     
===========================================
  Files          204      204              
  Lines         8047     8047              
  Branches      1409     1409              
===========================================
+ Hits          7602     7619      +17     
+ Misses         320      305      -15     
+ Partials       125      123       -2     

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