Missing documentation for Atlas

[wileyj]

Seems like this would be the best repo to address, since the atlas source code is here: https://github.com/stacks-network/stacks-blockchain/tree/master/src/net/atlas

Ideally, what I'd like to see is a markdown or something added here:https://github.com/stacks-network/stacks-blockchain/tree/master/docs explaining what Atlas is, and how it's designed in v2 (which can then be added to docs.stacks.co etc).

Reference:
#2013

The endpoints added by the PR should also be documented in https://github.com/stacks-network/stacks-blockchain/blob/master/docs/rpc-endpoints.md

[jcnelson]

Temporarily assigning to @wileyj to own this issue. Please feel free to re-assign!