docs: Document the boxes

Add a top-level `README.md` mapping the boxes and per-machine docs under
`docs/boxes/` (grouped `colony/`, `home/`, `misc/`), one file per host, VM and
container documenting role, services and networking with source pointers.

Also point `AGENTS.md` at the new docs.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/boxes/misc/kelder-spoder.md

@@ -0,0 +1,17 @@
+# kelder-spoder
+
+An nginx web host on the `kelder` site.
+
+- **Source:** [`kelder/containers/spoder/`](../../../nixos/boxes/kelder/containers/spoder)
+  (`default.nix`, `nginx.nix`)
+- **Host:** NixOS container on `kelder`
+
+## Role
+
+- Serves web content via **nginx** ([`nginx.nix`](../../../nixos/boxes/kelder/containers/spoder/nginx.nix)),
+  with ACME-managed certificates (nginx in the `acme` group, reloads on renewal).
+- Runs under the site's shared `kontent` user.
+
+## Networking
+
+- `internal` assignment (alt name `spoder-ctr`) on the kelder container network.