docs: add operator-facing jobq page, dedup coordinator.md pointer

hyperhive/hyperhive#2999 asked for an operator-facing page explaining
the job queue's node kinds in the abstract, without the coordinator's
implementation detail (leases, resource windows, module layout).

Add docs/jobq.md: what a queue row/step means, a plain-language table
of every node label an operator actually sees in the dashboard
(BU1LDS/R3BU1LD QU3U3 and swarm-ui's /jobs page both render the same
graph), and a rough shape for a typical rebuild.

Wire it into docs/README.md and the root CLAUDE.md reading-paths index
alongside coordinator.md. Add a pointer from coordinator.md's job-queue
section to the new page so the two don't try to explain the same thing
twice at different levels.

docs/README.md

@@ -79,7 +79,10 @@ declarations.
 
 ## Scheduler, CI, observability
 
-- **How does the rebuild queue work? What are queue kinds and
+- **What am I looking at in the build queue / R3BU1LD QU3U3? What does
+  each step mean?** → [`jobq.md`](jobq.md) — operator-facing, no
+  implementation detail.
+- **How does the rebuild queue work internally? What are queue kinds and
   sources?** → [`coordinator.md`](coordinator.md).
 - **How does the CI runner work? What's the auto-registration flow?** →
   [`ci.md`](ci.md).