docs: sort the top-level directories into five sections by what the reader wants

Every docs directory moves under 1-getting-started, 2-handbook,
4-implementation or 5-project; the options reference (section 3) comes from
the website and has no directory here. trust-boundary stays whole under the
handbook for now. web-ui, swarm and tools split between handbook and
implementation, and the command-line references move to
2-handbook/reference. Links, path references in code and nix, and the docs
index follow the moves; no page content changes.

hive-c0re/src/job_queue/mod.rs

@@ -23,7 +23,7 @@
 //! The queue is runtime-only (no persistence): an empty graph on boot; desired
 //! state is re-derived by the reconcile sweep. A single scheduler task
 //! ([`scheduler::run_worker`]) drives it; concurrency comes from the build-slot
-//! capacity, not multiple workers. Design: `docs/scheduler/coordinator.md::Job queue`.
+//! capacity, not multiple workers. Design: `docs/4-implementation/scheduler/coordinator.md::Job queue`.
 
 pub mod exec;
 pub mod model;