Expand nostrdb docs & mdbook

[alltheseas]

Summary

• 0b2bcff Add a real front door to README.md: platform requirements,
  quickstart, sample data workflow, and pointers to the new docs.
• e2c7c88 Introduce docs/getting-started.md with environment setup, build/
  test targets, fixture ingestion, a minimal C example, and troubleshooting
  tips.
• a732b86 Document nostrdb’s storage architecture—LMDB layout, packed notes/
  metadata, ingestion/query flow, and guidance for extending the engine safely.
• 56a7529 Create an API tour covering the major surfaces in src/nostrdb.h
  with code snippets for config, ingestion, filters, queries, and metadata
  builders.
• 8036743 Write a full ndb CLI guide detailing every command/flag, common
  workflows, and index-debug helpers.
• 54a9eda Catalog the generated C/Rust/Swift bindings, regeneration
  commands, and usage notes so consumers know what’s available today.
• ea1ec10 Stand up an mdBook site (docs/book/) that stitches all docs into
  a browsable book, plus README instructions for serving/building it.
• e0f85d5 Fix mdBook include paths and ignore the build artifacts so mdbook serve docs/book --open works cleanly.

[alltheseas]

Mdbook example screenshot