docs: add a grouped documentation index; fix references to now-local-only docs

Two concrete, verifiable documentation gaps from the open-source review:

- docs/ had no index. Adds docs/README.md grouping the 60-odd published docs by
  task — getting started, architecture, app development, design docs, ADRs,
  security, roadmap — in the bitcoin/bitcoin doc/ style the plan called for.
  Every link in it resolves (checked). The top-level README now points at it as
  the front door rather than duplicating the list.

- ROADMAP.md and tests/lifecycle/TESTING.md linked docs/multinode-testing-plan.md,
  which moved to local-only (it is a fleet node inventory, not published). Those
  references now describe the scope split in prose instead of pointing at a file
  that is not in the public tree.

The index is honest about what is missing: it names the three app-developer
docs the plan flagged as gaps (quadlet compilation, container lifecycle,
secrets materialisation) as not-yet-written, and points at the authoritative
code for each rather than pretending they exist.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

README.md

@@ -83,6 +83,9 @@ python3 scripts/check-app-catalog-drift.py --release --strict
 
 ## Documentation map
 
+The full, grouped index lives at **[docs/README.md](docs/README.md)**. The most
+common entry points:
+
 | Doc | Purpose |
 |-----|---------|
 | [Architecture](docs/architecture.md) | System layers, crates, data paths, security model |