Add cloud data WebSocket API

[towerofnix]

Between scratchfoundation/scratch-gui#3539 and related issues, we have enough info to add documentation about how to interface with cloud data by using WebSockets.

[joker314]

From a brief glance, this appears to use the same field names as the 2.0 protocol. Hence, here are some additional resources:

• Node.js implementation by trumank
• My understanding of the protocol by jokebookservice1 (me)

Both miss out the ability to create a new variable, which I think those 3.0 PRs expose!

[joker314]

Hmm, I was thinking of sending a PR for this, but all current APIs are about https://api.scratch.mit.edu. This API is about wss://clouddata.scratch.mit.edu. Is there a good place to put it?

EDIT: Also, since this API is already being used by 2.0, it doesn't strictly need to wait until the release of the 3.0 page.

[towerofnix]

It's sort of waiting on #46 - you shouldn't go forwards with this until we at least have some of that done as an example reference of writing style / etc. (No guarantees on when that will be done either - likely not within the very near future. Accordingly I won't assign you to this issue! :))

It will probably just go in an api/cloud.md document even though it's not documenting (the nonexistant) https://api.scratch.mit.edu/cloud.

[towerofnix]

See scratchfoundation/scratch-gui#3852 for reference on renaming/deleting variables.