Document the difference `PartialResource` vs `StreamResource` and `Datum` vs `StreamDatum`

[jacopoabramo]

This is a mirror of bluesky/bluesky#1980

The documentation currently shows only the schema implementation of an interface compatible with WritesExternalAssets (PartialResource and Datum), while there seems to be lacking information about the yielded documents from WritesStreamAssets (StreamResource and StreamDatum). As I've been encountering these interfaces in my application, I've had some troubles understanding what to insert in the yielded documents.

In particular, I'm curious of whether the use case of an Handler is still valid for the non-documented interface; as a non-control system user it would be interesting for me to experimenting with these as pseudo-devices that I can insert in my applications to inject somehow data writing while flying at a plan level.

In particular @coretl suggests that the second interface, WritesStreamAssets, should be preferred for new applications as it deals better with multiple flyers collecting with different rates.

Acceptance Criteria

• Add documentation section on "External stream assets"
• Evalute if the handler documentation (in particular for custom handlers) is still valid and applicable for external stream assets