docs: complete 2.0.0 changelog with all changes since beta.1

[benoitc]

Summary

• Add new 2.0.0 section documenting all changes since 2.0.0-beta.1
• Document new features: HTTP 1xx support, HTTPS proxy, proxy auth callback, on_connect_response callback, peercert function
• Document new options: auto_decompress, stream_to ownership, proxy_auth_fun, on_connect_response
• Document all bug fixes since beta.1 (15+ fixes)
• Add security section for header injection prevention and auth credential stripping
• Remove duplicates from beta.1 section (auto_decompress and stream_to were added post-beta)