Document uploading projects

[towerofnix]

I want to document https://projects.scratch.mit.edu (used in 3.0), not https://scratch.mit.edu/services/upload (used in 1.4 and offline 2.0) nor whatever online 2.0 uses.

At this point most relevant information is available, between scratchfoundation/scratch-storage#41 and scratchfoundation/scratch-gui#3021. (The scratch-projects PR mentioned in the scratch-gui issue is private, but it's for the backend code; we don't need to see it to know how to interface with it.)

[rschamp]

Online 2.0 uses the same projects.scratch.mit.edu that 3.0 will be using. The routes are just being renamed and so far are just aliases for each other.

[towerofnix]

POST https://projects.scratch.mit.edu (+ /projectID and PUT if updating) with parameter name sb3_file, filename sb3_file, body the .sb3 file (I think). Should be straightforward to test this. (I think it might already work, since I've seen some 3.0 projects on s.m.e, but not 100% sure.)

[towerofnix]

I haven't looked into this but there might be some changes incoming with Scratch 3.0, per scratchfoundation/scratch-gui#4015.