docs: add docs badge to readme (#370)

Author: kirit93

README.md

@@ -3,6 +3,7 @@
 [![License](https://img.shields.io/badge/License-Apache_2.0-blue)](https://github.com/NVIDIA/OpenShell/blob/main/LICENSE)
 [![PyPI](https://img.shields.io/badge/PyPI-openshell-orange?logo=pypi)](https://pypi.org/project/openshell/)
 [![Security Policy](https://img.shields.io/badge/Security-Report%20a%20Vulnerability-red)](SECURITY.md)
+[![Documentation](https://img.shields.io/badge/docs-latest-brightgreen)](https://docs.nvidia.com/openshell/latest/index.html)
 [![Project Status](https://img.shields.io/badge/status-alpha-orange)](https://docs.nvidia.com/openshell/latest/about/release-notes.html)
 
 OpenShell is the safe, private runtime for autonomous AI agents. It provides sandboxed execution environments that protect your data, credentials, and infrastructure — governed by declarative YAML policies that prevent unauthorized file access, data exfiltration, and uncontrolled network activity.