Document serialization API for panels.

[tim-schilling]

Fixes #1806

Checklist:

• I have added the relevant tests for this change.
• I have added an item to the Pending section of docs/changes.rst.

[matthiask]

The lint step fails with ../debug_toolbar/panels/__init__.py:docstring of debug_toolbar.panels.Panel.deserialize_stats:1: : Spell check: Deserialize: Deserialize stats coming from the store..

I don't know what's going on with MariaDB/MySQL.

[tim-schilling]

I don't think we're going to keep these API functions in. I'm discovering that I'd prefer to make the panel simply prepare the data in a serializable format rather than patch it in afterwards.

[tim-schilling]

Regarding mysql, I probably need to rebase on main.

[matthiask]

I'm discovering that I'd prefer to make the panel simply prepare the data in a serializable format rather than patch it in afterwards.

Good point, makes sense to me.

[tim-schilling]

Closing