* README: rewrite with feature overview, comparison, and tiered quick start
Expands the README with a fuller feature description, a "why this over
the alternatives" comparison against Keycloak/Authentik/Authelia/Zitadel,
an architecture diagram, and a three-tier quick start (unified theta-env
stack, standalone Docker, bare metal) instead of the old OpenLDAP-setup-
first structure.
* docs/index.md: fix stale setup.env quickstart snippet (.env -> setup.env, CFG_DOMAIN)