Add button to show/hide docstest prompts

[tristanlatr]

Fixes #390

Example: https://pydoctor--938.org.readthedocs.build/en/938/api/pydoctor.mro.html#c3_merge

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.33%. Comparing base (c1e68f8) to head (7cd6a6a).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #938   +/-   ##
=======================================
  Coverage   93.33%   93.33%           
=======================================
  Files          48       48           
  Lines        8912     8916    +4     
  Branches     1636     1637    +1     
=======================================
+ Hits         8318     8322    +4     
  Misses        337      337           
  Partials      257      257           

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[tristanlatr]

Next step is to add a button to copy the text

[github-actions]

Diff from pydoctor_primer, showing the effect of this PR on open source code:

sdk-python (https://github.com/temporalio/sdk-python): typechecking got 1.05x slower (221.2s -> 232.9s)
(Performance measurements are based on a single noisy sample)