expand nostrdb-rs docs, add mdbook

[alltheseas]

Summary

• e7b92a9 Rebuild the README with requirements/quickstart instructions, link
  into the nostrdb mdBook, add a Rust-specific guide (docs/rust.md), and ship
  a sample event fixture for examples/tests.
• 48deb93 Pull the README into crate-level docs and annotate the public API
  surface (Config, Filter, Ndb, Note, etc.) with module/type docs that
  reference the mdBook chapters.
• 6227e92 Add runnable ingest, query, and subscription examples that
  mirror the CLI workflows so users can exercise the Rust bindings end-to-end.