api: write `dvc.api.summon` doc

[jorgeorpinel]

Extracted from #908 (review)

The dvc.api.summon reference is pretty basic so far. It needs a more detailed description, double check parameter correctness and completeness, and some good examples.

[jorgeorpinel]

UPDATE: Per #908 (comment) I'm extracting the WIP on this to branch (api-summon) from where we can continue working this ticket. (Will probably need git merge master before retaking it.)

[jorgeorpinel]

Summon was never released 🤷🏼