docs: Obsidian-style vault under docs/

Add a navigable Obsidian vault as the project's first-class
technical documentation. Notes cross-reference with [[wikilinks]];
docs/index.md is the Map of Content.

New notes:
  index.md             MOC, entry point
  architecture.md      what the extension owns vs what lives outside
  data-model.md        entity-by-entity schema reference
  menu-tree.md         the arbitrary-depth tree concept
  order-flow.md        state machine + invoice listener + print
  nostr-layer.md       kinds 0/30402/5/1059, signing, t-tags
  api-reference.md     endpoint catalog by audience
  cms.md               Vue 3 + Quasar 2 UMD conventions, q-tree
  webapp-integration.md  multi-restaurant cart pattern + atomicity
  glossary.md          domain terms

Existing notes (kept as-is):
  adr-0001-menu-tree.md  the storage choice rationale
  design-conversation.md trimmed transcript

README.md adds a Documentation section pointing at docs/index.md
with the headline note list. Each note links to ~3-5 others; the
vault forms a connected graph.

A project-level memory rule (saved outside the repo) commits us to
keeping these docs in sync as the code evolves: any commit that
materially changes schema, API, order flow, Nostr surface, CMS
conventions, or webapp integration must update the relevant note(s)
in the same commit.

README.md

@@ -304,6 +304,20 @@ without the Nostr layer.
 - **Image upload pipeline** (today images are URLs; a CDN integration
   belongs in the AIO webapp, not here).
 
+## Documentation
+
+Deeper docs live in [`docs/`](docs/) as an Obsidian-style vault. Start
+at [`docs/index.md`](docs/index.md) (Map of Content) and follow the
+`[[wikilinks]]`. Highlights:
+
+- [`architecture`](docs/architecture.md) — layered overview
+- [`data-model`](docs/data-model.md) — every table, every relationship
+- [`menu-tree`](docs/menu-tree.md) — the tree as a concept
+- [`order-flow`](docs/order-flow.md) — state machine + payment + print
+- [`nostr-layer`](docs/nostr-layer.md) — kinds, tags, signing
+- [`webapp-integration`](docs/webapp-integration.md) — multi-restaurant cart pattern
+- [`adr-0001-menu-tree`](docs/adr-0001-menu-tree.md) — why adjacency + materialized path
+
 ## License
 
 MIT.