Document spaces

[aboodman]

Originally how Replicache apps were organized above the protocol was more open.

Now that we have settled into spaces being a more fundamental part of Replicache they should be documented that way. There needs to be a page for spaces, perhaps under "Understand Replicache".

Some of the things to communicate are:

• Spaces are not currently "part of" the Replicache protocol (the stuff represented in push/pull). They are "outside" Replicache in that sense.
• But in practice ~every application of Replicache uses this concept.
• Key constraints on spaces
• I also think it's worth reviewing rest of doc and making sure that spaces are discussed at appropriate times.
• Example data modeling: maybe one easy case (todos) and one hard case (discord)

[izakfilmalter]

Spaces confuse me. Would be great to go over some example spaces for common apps, uber / airbnb / gmail.

[aboodman]

This has been implemented: https://doc.replicache.dev/strategies/per-space-version