docs: fix search not working

[TomJGooding]

Fix the documentation search not working after a recent Read the Docs upgrade.

This adds sphinx_rtd_theme to the extensions as recommended in the install guide: https://sphinx-rtd-theme.readthedocs.io/en/stable/installing.html

Fixes: #3697

Type of changes

• Bug fix
• New feature
• Documentation / docstrings
• Tests
• Other

Checklist

• I've run the latest black with default args on new code.
• I've updated CHANGELOG.md and CONTRIBUTORS.md where appropriate.
• I've added tests for new code.
• I accept that @willmcgugan may be pedantic in the code review.

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.98%. Comparing base (225cb07) to head (a648cdc).
Report is 30 commits behind head on master.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3698      +/-   ##
==========================================
- Coverage   98.24%   97.98%   -0.27%     
==========================================
  Files          74       74              
  Lines        8105     8142      +37     
==========================================
+ Hits         7963     7978      +15     
- Misses        142      164      +22     

Flag	Coverage Δ
unittests	97.98% <ø> (-0.27%)	⬇️

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