docs: surface the Directory doc — register in-app, link from UI and site

docs/directory.md existed but was orphaned: not in the /docs registry,
not linked anywhere. Now:

- registered as /docs/directory ("Directory & Inventory")
- help icon on the Directory page header links to it (same pattern as
  users/groups/profile pages)
- linked from the docs site index feature list
- extended with the shared slug conventions (site_<name>, host_<hostname>),
  the automatic registration story (theta-env stack seeding, ldap-client
  Linux host enrollment), and the admin + read-only API surface (the
  read-only graph routes live at /api/discovery, not /api/directory).

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

docs/index.md

@@ -58,6 +58,7 @@ backend, that's the niche.
 - **All-in-one Docker image** — app + OpenLDAP + Redis in one container, or
   run the pieces separately via `app_*` env config.
 - **Geo-Location Scaling** — built-in support for N-Way Multi-Master OpenLDAP [replication](replication.html) across physical sites.
+- **[Directory & Inventory](directory.html)** — map sites, hosts, and services as a graph with rich metadata (IP/MAC, OS/kernel, ports, git repos), auto-provisioned access groups, and automatic registration from theta-env and ldap-client.
 
 ## Get it