Reorganize docs

[philippjfr]

Convert various user guides to Markdown and reorganizes the getting started guide.

[codecov]

Codecov Report

Merging #3875 (35133ab) into master (f4340a7) will decrease coverage by 0.05%.
The diff coverage is n/a.

❗ Current head 35133ab differs from pull request most recent head 5646332. Consider uploading reports for the commit 5646332 to get more accurate results

@@            Coverage Diff             @@
##           master    #3875      +/-   ##
==========================================
- Coverage   82.90%   82.85%   -0.06%     
==========================================
  Files         224      215       -9     
  Lines       32816    32369     -447     
==========================================
- Hits        27206    26818     -388     
+ Misses       5610     5551      -59     

Flag	Coverage Δ
ui-tests	34.38% <ø> (-0.30%)	⬇️
unitexamples-tests	74.93% <ø> (-0.16%)	⬇️

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

Impacted Files	Coverage Δ
panel/io/resources.py	86.64% <0.00%> (-0.95%)	⬇️
panel/io/state.py	69.15% <0.00%> (-0.20%)	⬇️
panel/io/server.py	74.60% <0.00%> (-0.05%)	⬇️
panel/auth.py	39.20% <0.00%> (ø)
panel/template/react/__init__.py	100.00% <0.00%> (ø)
panel/template/material/__init__.py	100.00% <0.00%> (ø)
panel/io/jupyter_server_extension.py	0.00% <0.00%> (ø)
panel/command/__init__.py
panel/pane/__init__.py
panel/widgets/__init__.py
... and 9 more

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more