Documentation: API functions

[jkiviluo]

In Mopo kick-off it was brought up that there should be better API function documentation. We have the documentation that comes from code comments (https://spine-toolbox.readthedocs.io/en/latest/autoapi/index.html), but there should also be a higher level description to make the API more understandable. Probably makes sense to do this for the new API only.

[soininen]

I think this is most critical for spinedb_api for development of tools/scripts that use Spine databases. Toolbox API is not that interesting unless you're actually developing the Toolbox itself.