Improve and document hold utility

[philippjfr]

The hold utility is likely criminally underused by users simply because we haven't made it very visible. In this PR I did a few things:

• Improve the way events are batched and dispatched by hold
• Add how-to guide on using hold
• Move it from panel.io.model to panel.io.document

[codecov]

Codecov Report

Attention: Patch coverage is 60.97561% with 16 lines in your changes missing coverage. Please review.

Project coverage is 82.29%. Comparing base (164cd2a) to head (665787e).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
panel/util/warnings.py	0.00%	7 Missing ⚠️
panel/io/document.py	76.92%	6 Missing ⚠️
panel/io/model.py	40.00%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #7474       +/-   ##
===========================================
+ Coverage   41.55%   82.29%   +40.74%     
===========================================
  Files         326      338       +12     
  Lines       48522    50851     +2329     
===========================================
+ Hits        20163    41848    +21685     
+ Misses      28359     9003    -19356     

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