Add WebSocket wire protocol documentation

[davidbrochart]

cc @jasongrout

[jasongrout]

Thanks! I added a few suggestions for clarification.

[codecov-commenter]

Codecov Report

Merging #693 (6858900) into main (6d2276c) will decrease coverage by 0.17%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #693      +/-   ##
==========================================
- Coverage   77.34%   77.17%   -0.18%     
==========================================
  Files         110      110              
  Lines       10542    10542              
  Branches     1427     1427              
==========================================
- Hits         8154     8136      -18     
- Misses       1988     2001      +13     
- Partials      400      405       +5     

Impacted Files	Coverage Δ
jupyter_server/services/contents/filemanager.py	68.61% <0.00%> (-1.61%)	⬇️
jupyter_server/services/kernels/handlers.py	56.76% <0.00%> (-1.10%)	⬇️
...ter_server/tests/services/contents/test_manager.py	96.01% <0.00%> (-0.94%)	⬇️
jupyter_server/serverapp.py	65.47% <0.00%> (-0.10%)	⬇️

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[davidbrochart]

Thanks @jasongrout !