refactor: move docs engine into iroh-docs (#2343)

## Description

Moves `iroh::docs_engine` to `iroh_docs::engine`. Most changes are
changed imports, dependencies, and visibility changes (plus docs where
they are now required). Everything is moved over, only the RPC
implementation stays in iroh for now.

The `iroh_docs::engine::Engine` is wrapped in a newtype `DocsEngine` in
iroh that derefs to the `Engine` so that we can implement the RPC
methods directly.

## Breaking Changes

* `iroh::client::docs::{Origin, SyncReason, SyncEvent}` are now
reexports of `iroh_docs::engine::{Origin, SyncReason, SyncEvent}`. There
is no need to change anything in your code that uses these structs via
`iroh::client::docs`.

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including how to migrate older code. -->

## Notes & open questions

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## Change checklist

- [x] Self-review.
- [x] Documentation updates if relevant.
- ~~[ ] Tests if relevant.~~
- [x] All breaking changes documented.