docs: rewrite README with structured overview and quick start guide

Replaces the minimal project description with a comprehensive README
including a component overview table, quick start instructions, common
Ansible operations, and links to detailed documentation. Aligns with
Red Panda Approval™ standards.

ansible/pplg/pgadmin.service.j2

@@ -0,0 +1,27 @@
+[Unit]
+Description=PgAdmin4 Web Interface (Gunicorn)
+After=network.target
+Wants=network.target
+
+[Service]
+Type=simple
+User={{pgadmin_user}}
+Group={{pgadmin_group}}
+WorkingDirectory=/usr/pgadmin4/web
+ExecStart=/usr/pgadmin4/venv/bin/python3 -m gunicorn pgAdmin4:app \
+    --bind 127.0.0.1:{{pgadmin_port}} \
+    --workers 1 \
+    --threads 4 \
+    --timeout 120 \
+    --access-logfile - \
+    --error-logfile -
+Restart=on-failure
+RestartSec=5
+NoNewPrivileges=true
+PrivateTmp=true
+StandardOutput=journal
+StandardError=journal
+SyslogIdentifier=pgadmin
+
+[Install]
+WantedBy=multi-user.target