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/forge/mod.rs

@@ -2,7 +2,7 @@
 //! every hive. The
 //! swarm-wide orgs, teams and repos, and each agent's read grant on
 //! `internal/docs`, are swarm-controller's. Full design:
-//! `docs/integrations/forge.md`.
+//! `docs/2-handbook/integrations/forge.md`.
 
 mod reader;