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>

docs/ROADMAP.md

@@ -49,9 +49,8 @@ security-first.
 ## 🔄 In progress
 
 - **Multinode pass** — run the same production gate across the whole test
-  fleet, plus cross-node federation/mesh suites
-  ([`multinode-testing-plan.md`](multinode-testing-plan.md)). This is the
-  current exit criterion.
+  fleet, plus cross-node federation/mesh suites. This is the current exit
+  criterion.
 - **Quadlet default flip** — flip the validated Quadlet path from opt-in to
   default fleet-wide; eliminates the last container-flapping root cause.
 - **Container-flapping elimination** — reconciler churn and failed-unit