Improved jupyter support

[forman]

• styled HTML output;
• refactorings in formatting;
• new PluginMeta.docs_url prop

Checklist (strike out non-applicable):

• Changes documented in CHANGES.md
• Related issue exists and is referred to in the PR description and CHANGES.md
• Added docstrings and API docs for any new/modified user-facing classes and functions
• Changes/features documented in docs/*
• Unit-tests adapted/added for changes/features
• Test coverage remains or increases (target 100%)

[codecov]

Codecov Report

Attention: Patch coverage is 99.13793% with 1 line in your changes missing coverage. Please review.

Project coverage is 99.44%. Comparing base (214542c) to head (872fd89).
Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
xrlint/formatters/simple.py	96.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #49      +/-   ##
==========================================
+ Coverage   99.43%   99.44%   +0.01%     
==========================================
  Files          65       65              
  Lines        2664     2719      +55     
  Branches      395      400       +5     
==========================================
+ Hits         2649     2704      +55     
  Partials       15       15              

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