docs: add pre-built agents section (Codebase QnA, CodeGen, Debugging)

[Harigithub11]

Adds three pre-built agent pages under the Developer > Pre-built Agents section in the sidebar:

• Codebase Q&A Agent — new page covering semantic search, API overview, use cases, and available tools
• CodeGen Agent — new page covering session-based code generation, diff reviews, and available tools
• Debugging Agent — restored and refactored page with 8-step methodology, dual-mode behavior, pre-implementation checklist, multi-agent mode, and linked tools reference

Also updates docs.json to add the Pre-built Agents navigation group under Developer.