docs: add Admin, webhook, and Projects guides

[biswaroop1547]

Summary

• add a focused Admin panel guide covering scope, controls, and audit boundaries
• add an Agent webhook guide covering setup, authentication, receipts, idempotency, and failure handling
• add a Projects feature guide and connect related context, memory, import, security, and API pages
• keep navigation alphabetized and add page icons

Checks

• npm run build
• npm run types:check
• cspell
• internal link, icon, and navigation checks