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/routes/docs.js

@@ -33,6 +33,7 @@ const DOCS = {
 	accounts:        {title: 'Accounts, Groups & Managers', file: path.join(__dirname, '../../docs/concepts-accounts.md')},
 	'oauth-apps':    {title: 'Connecting Apps (SSO)',   file: path.join(__dirname, '../../docs/concepts-oauth-apps.md')},
 	'api-tokens':    {title: 'API Tokens',              file: path.join(__dirname, '../../docs/concepts-api-tokens.md')},
+	directory:       {title: 'Directory & Inventory',   file: path.join(__dirname, '../../docs/directory.md')},
 
 	overview:        {title: 'Overview',        file: path.join(__dirname, '../../README.md')},
 	changelog:       {title: 'Changelog',       file: path.join(__dirname, '../../CHANGELOG.md')},