docs: restructure docs to resolve #14 and #11

Replace flat numbered layout with structured guides/modules layout:

- docs/README.md              — index with module/guide/diagram tables
- docs/getting-started.md     — quickstart (in-process, WebSocket, stdio)
- docs/architecture.md        — wire format, message taxonomy, sessions,
                                jobs, leases, threading, Jackson config
- docs/transports.md          — all transport adapters documented
- docs/conformance.md         — spec §§ coverage table and known gaps
- docs/troubleshooting.md     — all 15 error codes with causes and fixes
- docs/recipes.md             — idempotent retry, streaming, OTel, budgets,
                                delegation, resume, vendor extensions
- docs/guides/                — 11 topic guides (sessions, auth, jobs,
                                job-events, leases, credentials, delegation,
                                observability, resume, errors, vendor-extensions)
- docs/modules/               — 10 module reference pages

Delete 14 old numbered/flat docs files (00-overview.md, 02-concepts.md,
03-features/*, 05-reference/*).

Closes #14, closes #11

Author: nficano