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/tests.rs

@@ -554,7 +554,7 @@ fn rebuild_chain_declares_its_resources_on_the_brace() {
     // ⚠️ **The rename is a reversal, not tidying.** The old name asserted "the
     // slot follows the nix work and the lease follows the container" — each node
     // declaring what it personally needed. The brace inverts that for a
-    // coordinated subtree; see `docs/scheduler/coordinator.md`, _Braces_.
+    // coordinated subtree; see `docs/4-implementation/scheduler/coordinator.md`, _Braces_.
     //
     // (`hive_jobq` owns slot *fairness*, pinned there by
     // `a_contended_resource_goes_to_the_oldest_waiter`. What is c0re's is