Add disconnect warning to Notification

[philippjfr]

This PR enables a warning via the notification area that informs the user when the Websocket connection is dropped. The warning is enabled by default when the pn.config.notifications = True and the warning message can be configured.

Fix for #5240

• Add test
• Add documentation

[codecov]

Codecov Report

Merging #5244 (08ba4ee) into main (f2cefe3) will decrease coverage by 0.11%.
The diff coverage is 62.50%.

@@            Coverage Diff             @@
##             main    #5244      +/-   ##
==========================================
- Coverage   83.83%   83.72%   -0.11%     
==========================================
  Files         274      274              
  Lines       39606    39637      +31     
==========================================
- Hits        33203    33186      -17     
- Misses       6403     6451      +48     

Flag	Coverage Δ
ui-tests	40.37% <44.64%> (-0.19%)	⬇️
unitexamples-tests	73.64% <41.07%> (-0.14%)	⬇️

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Impacted Files	Coverage Δ
panel/io/admin.py	0.00% <0.00%> (ø)
panel/io/state.py	71.59% <0.00%> (-1.87%)	⬇️
panel/viewable.py	65.78% <0.00%> (ø)
panel/tests/ui/io/test_notifications.py	79.66% <71.42%> (-4.55%)	⬇️
panel/io/notifications.py	64.89% <75.00%> (+0.44%)	⬆️
panel/config.py	76.84% <78.57%> (-0.10%)	⬇️
panel/tests/test_server.py	99.34% <100.00%> (+<0.01%)	⬆️

... and 18 files with indirect coverage changes

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