WIP - sync streams beta docs

[benitav]

Starting this very WIP PR for our updated docs once we introduce Sync Streams in a beta release. The purpose for now is to start fleshing out how we'll document them, and identify gaps.

At a high level, we're introducing Sync Streams docs as a standalone section primarily for users just starting out with PowerSync, and not knowing/caring about Sync Rules.

For existing users, we're keeping the Sync Rules docs section largely as is as a legacy docs section.

The structure is currently as follows

usage/
|- sync-streams/                   # New Sync Streams
|  |- quick-start.mdx
|  |- reference/
|  |  |- query-syntax.mdx
|  |  |- operators-and-functions.mdx
|  |  |- types.mdx
|  |  |- case-sensitivity.mdx
|  |  |- client-primary-key.mdx
|  |  └── joins.mdx
|  |- advanced/
|  |  |- compatibility.mdx
|  |  |- multiple-client-versions.mdx
|  |  |- schemas-and-connections.mdx
|  |  |- partitioned-tables.mdx
|  |  └── shared-databases.mdx
|  |- under-the-hood.mdx
|  └── glossary.mdx
|- sync-rules/                     # Legacy Sync Rules
|  |- ... (current structure)

Still to do:

• there are a bunch of subsections marked as todo that need to be adapted for sync streams