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/oauth2-proxy-prometheus.service.j2

@@ -0,0 +1,18 @@
+[Unit]
+Description=OAuth2-Proxy for Prometheus UI
+After=network.target prometheus.service
+Wants=prometheus.service
+
+[Service]
+Type=simple
+ExecStart=/usr/local/bin/oauth2-proxy --config={{prometheus_oauth2_proxy_dir}}/oauth2-proxy.cfg
+Restart=on-failure
+RestartSec=5
+NoNewPrivileges=true
+PrivateTmp=true
+StandardOutput=journal
+StandardError=journal
+SyslogIdentifier=oauth2-proxy-prometheus
+
+[Install]
+WantedBy=multi-user.target