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/openwebui/openwebui.service.j2

@@ -0,0 +1,19 @@
+[Unit]
+Description=Open WebUI
+After=network.target
+
+[Service]
+Type=simple
+User={{openwebui_user}}
+Group={{openwebui_group}}
+WorkingDirectory={{openwebui_directory}}
+EnvironmentFile={{openwebui_directory}}/.env
+ExecStart={{openwebui_directory}}/.venv/bin/open-webui serve --port {{openwebui_port}}
+Restart=always
+RestartSec=3
+StandardOutput=journal
+StandardError=journal
+SyslogIdentifier=openwebui
+
+[Install]
+WantedBy=multi-user.target