Asset Catalogs: add documentation for undo system

Merge remote-tracking branch 'origin/master' into temp-asset-browser-catalog-undo

Asset Catalogs: undo/redo working (on service level)

Still needs named snapshots.

Asset Catalogs: undo stack for catalog edits

Add an undo stack for catalog edits. Still work in progress:
- Undo/redo behaviour needs to be refined (or test adjusted to match current behaviour)
- Stored snapshots need to have a name, so it can be shown in the UI what will be undone/redone.

Implement `AssetCatalogCollection::deep_copy()`

Implement a deep copy function on `AssetCatalogCollection` that can be
used to create snapshots for storage in an undo stack.

Cleanup: Asset Catalogs, make separate structure for a catalog collection

The collection of catalogs, as managed by a single instance of
`blender::bke::AssetCatalogService`, is now stored in a separate class
`AssetCatalogCollection`.

This opens the door to storing & restoring deep copies of that class,
for an upcoming undo system.