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.

nix/host-modules/hive-forge/default.nix

@@ -161,7 +161,7 @@ in
   # netns, on the one host with `deploy.forgejo.enable`. Agents reach it
   # at `forge.<domain>` via the gateway. State at
   # `/var/lib/nixos-containers/hive-forge/var/lib/forgejo/` survives
-  # restart. See `docs/networking/gateway.md::hive-forge container shape`.
+  # restart. See `docs/4-implementation/networking/gateway.md::hive-forge container shape`.
 
   imports = [
     ./service.nix
@@ -319,7 +319,7 @@ in
     # `server_name = forge.domain`, proxies all `/` → forgejo. Tuned for
     # git: `client_max_body_size 1G`, `proxy_read_timeout 1h` (multi-GB
     # clones). SSH stays direct on `forge.sshPort`. See
-    # `docs/networking/gateway.md`.
+    # `docs/4-implementation/networking/gateway.md`.
     services.hyperhive.gateway.vhosts = {
       "${cfg.domain}" = {
         locations = {
@@ -963,7 +963,7 @@ in
               # container, where forgejo already stores it at rest in the
               # login-source row — root and forgejo are the only
               # principals here, and root can read the resting copy
-              # anyway. Accepted deliberately (see docs/swarm/), not
+              # anyway. Accepted deliberately (see docs/2-handbook/swarm/), not
               # overlooked; upstream gap filed.
               # An ARRAY, not a string. A string of arguments has to be
               # word-split at the call site to become argv, and splitting