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[bot]]

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 :warning:
panel/io/document.py	76.92%	6 Missing :warning:
panel/io/model.py	40.00%	3 Missing :warning:

Additional details and impacted files

```diff @@ 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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