Document v2 api contract

[slimslenderslacks]

I created some placeholder markdown files here:

https://github.com/atomisthq/docs/tree/main/docs/authoring/api

I also added some markdown for transactions. This is what it looks like on staging: https://docs.atomist.services/authoring/api/transactions/

@neilprosser - do you think this is an okay template for documenting these calls?