docs: mintlify documentation revamp

[Harigithub11]

Summary

• Revamped documentation structure and content across agents, API reference, build flow, concepts, and extensions
• Added tutorials section with quickstart improvements and next steps navigation
• Updated images, removed deprecated pages, and improved overall consistency

Changes

• Updated agent pages (QnA, Code, Debug, Tools Reference)
• Refreshed API reference docs and OpenAPI specs
• Added tutorials: explore codebase, make code changes, generate API key
• Improved quickstart with next steps and external link handling
• Added new concept and extension pages
• Cleaned up self-hosting and custom agents sections