Add documentation for built in dependencies

[julianstirling]

Talking about the cancel hook in the Microscope Repository, we have mentioned it needs to be documented. Probably the place to do this is in a section tutorial about writing actions.

If there was something about "Built in dependencies".

Any of your actions can use a number of dependencies that are built in to labthings-fast API. These include:

InvocationLogger

A logger for sending messages specific to a specific run of this action.

some example code which includes logging

CancelHook

A dependency that allows external tasks to cancel this action

some example code which use both ways of raising the cancel error, or uses one and comments about the other.

[rwb27]

I think this is addressed by the big docstring push. There's a section at the bottom of the dependencies page that links to the convenience-import deps module. A tutorial on writing actions is a nice idea, currently the tutorial stops after "how to run LabThings" but clearly there should be a page that does this.

[rwb27]

This will be closed by #194 migrating away from dependencies.

[rwb27]

Closed by #194