docs: fix stale multi-site claims, document LDAP MMR auto-config + CFG_PUBLIC_DOMAIN

The published docs site had drifted from what actually shipped:

- docs/sso/multi-site.md said the no-inbound relay was "designed but
  not automated" -- it's been automated since earlier this session.
  Also said "don't combine" multi-site join and LDAP replication --
  they're integrated now (join auto-configures LDAP MMR).
- docs/sso/replication.md (the actually-published/linked replication
  page -- distinct from theta-directory's own docs/replication.md,
  which isn't linked from this site's nav at all) still only described
  the old fully-manual LDAP_SERVER_ID/LDAP_REPLICATION_HOSTS setup,
  with zero mention of the new auto-config or CFG_LDAP_MMR_MANUAL.
- docs/index.md's feature bullet described multi-site purely as "N-Way
  Multi-Master LDAP replication" -- the actual master/spoke join
  feature (the more commonly-used, higher-level mechanism) wasn't
  mentioned on the homepage at all.
- CFG_PUBLIC_DOMAIN (shipped, never documented anywhere an operator
  would read it) now explained in multi-site.md.
- spoke.env now discoverable from quickstart.md, not just multi-site.md.

Also documents the real, load-bearing limitation from this session's
promotion/LDAP-orphan fix: the master's own replication peer list only
updates on its own next setup.sh run, not live the instant a spoke
joins or a promotion happens.

docs/quickstart.md

@@ -74,6 +74,11 @@ file shape.
 > exist, `./setup.sh` migrates them into `./config/` preserving your existing
 > secrets — no need to write a `setup.env`.
 
+> **Joining an existing Theta Directory cluster instead of seeding a fresh
+> one?** See [Multi-Site (Master/Spoke Join)](sso/multi-site.html) —
+> `spoke.env.example` has the join-a-cluster vars split out into their own
+> file, or set them directly in `setup.env` (which has every option).
+
 ---
 
 ## 3. Run