Remove deprecated headers and standardize README

[pirate]

Summary

This PR updates the Rust SDK repository to align with the main Stagehand project's standardized branding and removes deprecated header configurations from example code.

Changes

• README Enhancements: Added comprehensive Stagehand branding header matching the main repository, including:

  • Logo with dark/light theme support
  • Links to documentation (updated to /v3/sdk/rust)
  • License and Discord badges
  • Trendshift badge
  • Director.ai integration callout
  • "What is Stagehand?" and "Why Stagehand?" sections for better context

• Examples Directory: Created new examples/ directory with basic.rs demonstrating:

  • Complete SDK usage workflow (connect, start, navigate, observe, act, extract, execute)
  • Proper initialization using model_name instead of deprecated model_provider/model_api_key headers
  • Real-world example: Navigating Hacker News, finding top post comments, extracting data, and using autonomous agent
  • All streaming API patterns (act, observe, extract, execute)
  • Structured data extraction with JSON schemas and Serde
  • No deprecated headers - uses modern V3Options with Model enum

• Documentation URL: Updated docs link from generic path to /v3/sdk/rust for Rust-specific documentation

• Code Quality: Preserved ALPHA notice and all existing README content while adding standardized header

Test plan

• Verify README renders correctly on GitHub with both light and dark themes
• Confirm all badges and links are functional
• Check that example code in examples/basic.rs compiles without warnings
• Validate that example uses only non-deprecated API patterns
• Ensure documentation link points to correct Rust SDK docs

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