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/index.md

@@ -74,10 +74,12 @@ The SSO Manager and the proxy it fronts, both stood up by one `./setup.sh` run:
   management built directly into the web interface, allowing operators to drive
   system administration and automation pipelines programmatically without an
   active browser session.
-- **Multi-Site Geo-Replication**: Built-in support for N-Way Multi-Master LDAP
-  replication, allowing directory states to sync across geographically separated
-  physical hardware or remote data centers for high availability and low-latency
-  local reads.
+- **Multi-Site Geo-Replication**: A master site and any number of spokes join
+  with one key and one URL, staying in sync automatically (no manual LDAP
+  config) — see [Multi-Site (Master/Spoke Join)](sso/multi-site.html). Raw
+  N-Way Multi-Master LDAP replication is also available directly for
+  deployments that want every site independently writable with no
+  master/spoke concept — see [Geo-Location Scaling](sso/replication.html).
 - **Multi-Target Load Balancing**: Native reverse-proxy load balancing that
   distributes traffic across multiple application backends using customizable
   health checks and round-robin strategies.