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.

First, examine the existing Rust client documentation hierarchy to understand the structure to mirror. Then, locate the appropriate sections in the docs repository (likely under Smart Contracts / Tools) where the standard crates pages should be added. For each crate (miden-lib, miden-objects, miden-tx), decide whether to ingest existing doc comments or author new content, and create follow-up issues for sparse documentation in the source repos.

Rédigé par le modèle d'indexation à partir du texte de l'issue.