docs: add DeepWiki badge for AI-powered project documentation

[devv-shayan]

What

This PR adds the official “Ask DeepWiki” badge to the top of our README.

Why

DeepWiki is an AI-powered docs layer that continuously indexes your public GitHub repo to generate searchable, wiki-style pages, interactive Q&A, and visual architecture diagrams—all without any manual upkeep. Adding this badge unlocks:

• Instant AI docs: One click takes you to DeepWiki’s structured documentation and module overviews.
• Interactive Q&A assistant: Ask questions (“How does tracing work?”) and receive code-aware answers.
• Visual architecture diagrams: Auto-rendered flowcharts and dependency graphs for at-a-glance clarity.
• Zero-maintenance updates: DeepWiki’s crawler auto-syncs docs on every commit or weekly by default.
• Free for open-source: No cost to use DeepWiki on public repositories.
• Effortless setup: Activate by replacing github.com with deepwiki.com—no installs or config needed.
• Reduced onboarding time: New contributors ramp up faster via an always-fresh, AI-driven docs portal.

Changes

• Inserted the markdown badge at the top of the README:

  [![Ask DeepWiki](https://deepwiki.com/badge.svg)](https://deepwiki.com/openai/openai-agents-python)