docs: add PyPI, Python version, downloads, and docs badges to README

Merge #6023

## Summary

Add shields.io badges to the README to improve project discoverability and provide quick metadata at a glance.

### Badges added:
- **PyPI version** — links to the [google-adk](https://pypi.org/project/google-adk/) package page
- **Python versions** — shows supported Python versions (3.10+)
- **Monthly downloads** — displays PyPI download count
- **Documentation** — links to the official docs at [adk-docs](https://google.github.io/adk-docs/)

### Before
Only the License badge was present.

### After
README header now shows License, PyPI version, Python versions, downloads, and docs badges — consistent with other Google open-source Python projects.

Co-authored-by: George Weale <gweale@google.com>
PiperOrigin-RevId: 930805673

Author: GWeale

README.md

@@ -1,6 +1,10 @@
 # Agent Development Kit (ADK) 2.0
 
 [![License](https://img.shields.io/badge/License-Apache_2.0-blue.svg)](LICENSE)
+[![PyPI version](https://img.shields.io/pypi/v/google-adk.svg)](https://pypi.org/project/google-adk/)
+[![Python versions](https://img.shields.io/pypi/pyversions/google-adk.svg)](https://pypi.org/project/google-adk/)
+[![PyPI downloads](https://img.shields.io/pypi/dm/google-adk.svg)](https://pypi.org/project/google-adk/)
+[![Docs](https://img.shields.io/badge/docs-latest-blue.svg)](https://google.github.io/adk-docs/)
 
 <h2 align="center">
   <img src="https://raw.githubusercontent.com/google/adk-python/main/assets/agent-development-kit.png" width="256"/>