add collaboration about qboard

[pihart]

From #6:

• Add comments, annotations, or notes for objects or object groups. These do not get exported to PDF but they are stored in the JSON and shared for collaboration. Probably should only be bindable but it can also be in the context menu or as a floating circle a la Google Docs.

[pihart]

Make sure to follow #34 (comment)

[pihart]

I can probably take 2-6 in a few weeks

[cjquines]

note that @jchen was also planning to work on this

i am very strongly opinionated about how 2 and 3 should be done, so if you do plan to work on this you should probably merge that as soon as you write 2 and 3

[pihart]

When generating a link for spectator mode, provide an option to lock the document, meaning any future edits (including by the sharer) are not reflected on the link (essentially you're just hosting the static qboard json file, and maybe the prior history stacks).

This will be very useful for virtual exams, where students need to show work or provide diagrams. Instead of uploading the json file, they enter the qboard hosted id.