Add source-owned note transport docs

[BrianSeong99]

Summary

Adds source-owned Note Transport docs under docs/external/src and fixes the Docker Compose persistence path used by the node preview stack.

What changed

• Adds overview, design, operator, and user docs for the note transport service.
• Documents the current gRPC API, cursor model, retention/storage behavior, telemetry env vars, and current block-context limitations.
• Updates make book to point contributors at the docs-repo ingestion workflow instead of the removed mdBook path.
• Passes --database-url /app/data/node.db in Docker Compose so the mounted node_data volume is actually used, and removes the non-existent HTTP health port mapping.

Why

PR #85 added a standalone Docusaurus app and included docs that drifted from the current implementation. This keeps the source of truth in the service repo while letting 0xMiden/docs ingest the pages into docs.miden.xyz.

Verification

• docker compose -f bin/node/docker/docker-compose.yml --project-directory . config
• Docker image build from bin/node/docker/Dockerfile
• Container runtime smoke test with --database-url /app/data/node.db: node started, gRPC port opened, migrations ran, and the SQLite notes table was created
• git diff --check

Notes

cargo fmt --check currently fails on pre-existing Rust formatting drift outside this docs/Docker change.