Improve markdown rendering

[philippjfr]

Improve Markdown renderering by using markdown-it-py as default.

• Fixes Improve Markdown Experience #4149
• Anchor links

• Code sections

• Tables

• Code blocks

• Add code copy button
• Strikethrough

[codecov]

Codecov Report

Merging #4688 (e6cc618) into main (bb9a7be) will decrease coverage by 0.03%.
The diff coverage is 79.36%.

@@            Coverage Diff             @@
##             main    #4688      +/-   ##
==========================================
- Coverage   83.16%   83.13%   -0.03%     
==========================================
  Files         266      266              
  Lines       37453    37488      +35     
==========================================
+ Hits        31149    31167      +18     
- Misses       6304     6321      +17     

Flag	Coverage Δ
ui-tests	39.62% <58.73%> (-0.03%)	⬇️
unitexamples-tests	73.24% <53.96%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
panel/pane/markup.py	89.77% <65.62%> (-4.04%)	⬇️
panel/tests/ui/io/test_server.py	73.52% <66.66%> (ø)
panel/tests/pane/test_markup.py	99.38% <100.00%> (+0.01%)	⬆️
panel/tests/test_interact.py	99.00% <100.00%> (ø)
panel/tests/ui/io/test_state.py	72.22% <100.00%> (ø)
panel/tests/ui/pane/test_markup.py	100.00% <100.00%> (ø)
panel/tests/ui/template/test_bootstraptemplate.py	100.00% <100.00%> (ø)
panel/tests/ui/template/test_fastgridtemplate.py	100.00% <100.00%> (ø)
panel/tests/ui/template/test_fastlisttemplate.py	100.00% <100.00%> (ø)
panel/tests/ui/template/test_materialtemplate.py	100.00% <100.00%> (ø)
... and 1 more

... and 3 files with indirect coverage changes

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