Add standard crates documentation section mirroring Rust client hierarchy

Source: DevEx Sync (Notion).

The "standard crates" pages were called out as still needed. The intended shape is to mimic the Rust client's documentation hierarchy (initialization → account APIs → transactions → notes), giving each standard crate (e.g. `miden-lib`, `miden-objects`, `miden-tx`) its own clear API hierarchy in the docs.

Scope:

- Author standard-crates landing page + per-crate sections under the appropriate Smart Contracts / Tools area.
- Mirror the structure Brian has endorsed in the Rust client docs.
- Decide ingestion vs. locally authored on a crate-by-crate basis.

Upstream dependency: where doc comments are sparse, file follow-ups against the source repos (likely in `protocol`) — but the IA + docs framing is owned here.