feature(rss): add option to set a stylesheet

[Guts]

It comes with a special value to use the XSL shipped with the plugin

[codecov]

Codecov Report

❌ Patch coverage is 73.33333% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.54%. Comparing base (46d2f0f) to head (ac4fb39).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
mkdocs_rss_plugin/plugin.py	69.23%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #432      +/-   ##
==========================================
- Coverage   83.74%   83.54%   -0.20%     
==========================================
  Files          11       11              
  Lines         769      784      +15     
  Branches      129      132       +3     
==========================================
+ Hits          644      655      +11     
- Misses         82       84       +2     
- Partials       43       45       +2     

Flag	Coverage Δ
unittests	83.54% <73.33%> (-0.20%)	⬇️

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Files with missing lines	Coverage Δ
mkdocs_rss_plugin/config.py	100.00% <100.00%> (ø)
mkdocs_rss_plugin/models.py	100.00% <100.00%> (ø)
mkdocs_rss_plugin/plugin.py	89.50% <69.23%> (-1.77%)	⬇️

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[sonarqubecloud]

Quality Gate passed

Issues
1 New issue
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud