Reorganize docs and convert static notebooks to markdown

[philippjfr]

Previously we used notebooks for all our user guides. However a lot of the docs are actually static, i.e. almost entirely Markdown and since we have a lot more power if we can leverage MyST flavored markdown in standard .md files this seems like a better approach.

[codecov]

Codecov Report

Merging #3833 (6f0c7d5) into master (6083cd7) will decrease coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3833      +/-   ##
==========================================
- Coverage   82.94%   82.90%   -0.05%     
==========================================
  Files         222      222              
  Lines       32368    32430      +62     
==========================================
+ Hits        26847    26885      +38     
- Misses       5521     5545      +24     

Flag	Coverage Δ
ui-tests	33.90% <ø> (-0.27%)	⬇️
unitexamples-tests	75.34% <ø> (-0.08%)	⬇️

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Impacted Files	Coverage Δ
panel/tests/ui/io/test_jupyter_server_extension.py	40.74% <0.00%> (-59.26%)	⬇️
panel/tests/conftest.py	86.89% <0.00%> (-1.32%)	⬇️
panel/util.py	86.41% <0.00%> (-0.76%)	⬇️
panel/widgets/tables.py	88.36% <0.00%> (-0.13%)	⬇️
panel/reactive.py	78.31% <0.00%> (-0.03%)	⬇️
panel/tests/widgets/test_tables.py	99.66% <0.00%> (+<0.01%)	⬆️
panel/tests/ui/widgets/test_tabulator.py	98.88% <0.00%> (+0.02%)	⬆️

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