docs(configuration): align env var reference with current backend

Bring docs/getting-started/configuration.mdx up to date with the
v0.72.0 backend:

- Remove PORT from the optional env vars table. The listen port is
  hardcoded to 1852 in backend/src/helpers/constants.ts and is never
  read from the environment. Replace it with a Listen port section
  that explains the fixed port and host-port remapping.
- Document API_RATE_LIMIT (default 200) and API_POLLING_RATE_LIMIT
  (default 300), both applied in production only.
- Note the /app/compose fallback default for COMPOSE_DIR while still
  pointing readers at the 1:1 path rule.
- Point the SSO env var section at the new SSO Quickstart page and
  keep the SSO feature reference as the deeper dive.
- Tighten First boot and cross-link to Quickstart so the screenshot
  is not duplicated.
- Add a Where to next CardGroup matching the refreshed Introduction
  and Quickstart pages.

Drop stale PORT=1852 and JWT_SECRET=your-secure-jwt-secret-here
lines from .env.example so the example no longer contradicts the
docs (PORT is hardcoded, JWT_SECRET is auto-generated and persisted
to the database during initial setup).

.env.example

@@ -1,15 +1,9 @@
 # Sencho Configuration
 # Copy this file to .env and update the values for production
 
-# JWT secret - generate a secure random string for production
-JWT_SECRET=your-secure-jwt-secret-here
-
 # Directory containing docker-compose files
 COMPOSE_DIR=/path/to/your/compose/files
 
-# HTTP server port (default: 1852)
-PORT=1852
-
 # Database and state directory inside the container (default: /app/data)
 DATA_DIR=/app/data