doc: start an architecture document with a high-level description of the crates

ARCHITECTURE.md

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+# Architecture
+
+The SDK is split into multiple layers:
+
+```
+    WASM (external crate matrix-rust-sdk-crypto-wasm)
+      /
+     /     uniffi
+    /     /
+   /     bindings (matrix-sdk-ffi)
+ crypto   |
+bindings  |
+   |      |
+   |     UI (matrix-sdk-ui)
+   |      \
+   |       \
+   |   main (matrix-sdk)
+   | /     /
+crypto    /
+     \   /
+      store (matrix-sdk-base, + all the store impls)
+        |
+      common (matrix-sdk-common)
+```
+
+Where the store implementations are `matrix-sdk-sqlite` and `matrix-sdk-indexeddb` as well as
+`MemoryStore` which is defined in `matrix-sdk-base`.
+
+## `crates/matrix-sdk`
+
+This is the main crate, and one that is expected to be used by most consumers. Notable data types
+include:
+
+- the `Client`, which can run room-independent requests: logging in/out, creating rooms, running
+  sync, etc.
+- the `Room`, which represents a room and its state (notably via the observable `RoomInfo`), and
+  allows running queries that are room-specific, notably sending events.
+
+## `crates/matrix-sdk-base`
+
+A *sans I/O* crate to represent the base data types persisted in the SDK. No network or storage I/O
+happens in this crate, although it defines traits (`StateStore` and `EventCacheStore`) representing
+storage backends, as well as dummy in-memory implementations of these traits.
+
+## `crates/matrix-sdk-common`
+
+Common helpers used by most of the other crates; almost a leaf in the dependency tree of our own
+crates (the only crate it's using is test helpers).
+
+## `crates/matrix-sdk-crypto`
+
+A *sans I/O* implementation of a state machine that handles end-to-end encryption for Matrix
+clients. It defines a `CryptoStore` trait representing storage backends that will perform the
+actual storage I/O later, as well as a dummy in-memory implementation of this trait.
+
+## `crates/matrix-sdk-indexeddb`
+
+Implementations of `EventCacheStore`, `StateStore` and `CryptoStore` for a
+indexeddb backend (for use in Web browsers, via WebAssembly).
+
+## `crates/matrix-sdk-qrcode`
+
+Implementation of QR codes for interactive verifications, used in the crypto crate.
+
+## `crates/matrix-sdk-sqlite`
+
+Implementations of `EventCacheStore`, `StateStore` and `CryptoStore` for a
+SQLite backend.
+
+## `crates/matrix-sdk-store-encryption`
+
+Low-level primitives for encrypting/decrypting/hashing values. Store implementations that
+implement encryption at rest can use those primitives.
+
+## `crates/matrix-sdk-ui`
+
+Very high-level primitives implementing the best practices and cutting-edge Matrix tech:
+
+- `EncryptionService`: a specialized service running simplified sliding sync (MSC4186) for
+  everything related to crypto and E2EE for the current `Client`.
+- `RoomListService`: a specialized service running simplified sliding sync (MSC4186) for
+  retrieving the list of current rooms, and exposing its entries.
+- `SyncService`: a wrapper for the two previous services, coordinating their running and shutting
+  down.
+- `Timeline`: a high-level view for a `Room`'s timeline of events, grouping related events
+  (aggregations) into single timeline items.
+
+## `bindings/matrix-sdk-crypto-ffi/`
+
+FFI bindings for the crypto crate, used in a Web browser context via WebAssembly. These use
+`wasm-bindgen` to generate the bindings. These bindings are used in Element Web and the legacy
+Element apps, as of 2024-11-07.
+
+## `bindings/matrix-sdk-ffi/`
+
+FFI bindings for important concepts in `matrix-sdk-ui` and `matrix-sdk`, generated with
+[UniFFI](https://github.com/mozilla/uniffi-rs) and to be used from other languages like
+Swift/Go/Kotlin. These bindings are used in the ElementX apps, as of 2024-11-07.
+
+## `bindings/matrix-sdk-ffi-macros/`
+
+Macros used in `bindings/matrix-sdk-ffi`.
+
+## `testing/matrix-sdk-test/`
+
+Common test helpers, used by all the other crates.
+
+## `testing/matrix-sdk-test-macros/`
+
+Implementation of the `#[async_test]` test macro.
+
+## `testing/matrix-sdk-integration-testing/`
+
+Fully-fledged integration tests that require spawning a Synapse instance to run. A docker-compose
+setup is provided to ease running the tests, and it is compatible for running with Podman too.
+
+# Inspiration
+
+This document has been inspired by the reading of this [blog post](https://matklad.github.io/2021/02/06/ARCHITECTURE.md.html).