Docs: Document MemoryBuilder  #584

Description

Add documentation for MemoryBuilder (packages/adk/src/unified-memory/). Should explain how it wraps SessionService and MemoryService into a single entry point (similar to AgentBuilder), cover addMessage/recall with the SimpleMessage format, working memory with templates via session state, configurable lastMessages, optional MemoryService for search, and how to swap in DatabaseSessionService or vector-backed storage. Include a walkthrough based on the 11-unified-memory example. Goal is to show users they don't need to wire two services manually.