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>

nodejs/views/directory.ejs

@@ -29,6 +29,7 @@
                 <div class="card-header actionMessage" style="display:none"></div>
                 <div class="p-3 pb-0 text-muted small border-bottom">
                     <i class="fa-solid fa-circle-info"></i> Manage infrastructure, services, and their relationships.
+                    <a href="/docs/directory" class="text-reset float-end" title="Help"><i class="fa-solid fa-circle-question"></i></a>
                 </div>
 				<div class="table-responsive">
 				<table class="card-body table table-striped mb-0">