Search bar improvments and Help page

[tristanlatr]

Implement one idea described in #822, more specifically:

• add a leading wildcard to the query when the term contains a dot ".".

Fixes #819

Other changes:

• Move the NotFoundLinker to the linker.oy module because t's now used by the help page renderer.
• Simplify the readthedocs theme and fix an issue that prevented to use the search bar from the module index page and other summary pages.

[tristanlatr]

This could be a good opportunity to fix the search bar with the readthedocs in the summary pages. Which is simply not working at the moment :/

[codecov]

Codecov Report

Attention: Patch coverage is 96.29630% with 1 line in your changes missing coverage. Please review.

Project coverage is 92.30%. Comparing base (ad1fc12) to head (60cb87c).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
pydoctor/linker.py	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #823      +/-   ##
==========================================
+ Coverage   92.29%   92.30%   +0.01%     
==========================================
  Files          47       47              
  Lines        8514     8540      +26     
  Branches     1695     1696       +1     
==========================================
+ Hits         7858     7883      +25     
- Misses        385      386       +1     
  Partials      271      271              

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[tristanlatr]

This PR is ready to be merged.

[github-actions]

According to pydoctor_primer, this change doesn't affect pydoctor warnings on a corpus of open source code. ✅