Adds a Lists documentation page to the Core Concepts section covering overview, list types, and SDK examples for all CRUD operations.

Summary by cubic

Adds a new Core Concepts page for Lists to explain allow/block filtering for sending and receiving emails. Includes Python and TypeScript examples for listing, creating, getting, and deleting entries.

• New Features
  • New "Lists" docs page covering list types: send/receive × allow/block.
  • Notes that entries can be full emails or domains; block lists support an optional reason.
  • SDK examples for list, create, get, and delete in Python and TypeScript.
  • Navigation updated with Lists entry and filter icon.

^{Written for commit 15a6ed8. Summary will update on new commits.}