docs: concept guides and complete API reference

[AmitoVrito]

Summary

• Add 4 concept guides: RAG, Agents, Graph Workflows, Embeddings (closes Concept guide — How RAG works (embeddings, chunking, retrieval) #65, Concept guide — How agents and tools work #66, Concept guide — How graph workflows work #67, Concept guide — Embeddings and vector search #68)
• Add 6 new API reference pages: RAGPipeline, StateGraph, Agents, Memory, Evaluation, Observability (closes API reference — RAGPipeline and RAGConfig (complete) #69, API reference — StateGraph and CompiledGraph (complete) #70, API reference — All Agent classes (complete) #71, API reference — All memory backends (complete) #72, API reference — Evaluation and Observability classes (complete) #74)
• Expand Retriever API reference from 1 strategy to 16 with selection guide (closes API reference — All 20 retrieval strategies (complete parameter tables) #73)
• Update sidebars.ts: add Concepts category and expand API Reference section to 9 items

Files added

• docs/concepts/rag.md — RAG pipeline concept guide with ASCII diagram, chunking, HNSW, hybrid search
• docs/concepts/agents.md — ReAct loop, function calling, multi-agent patterns
• docs/concepts/graphs.md — StateGraph DAG model, reducers, HITL, checkpointing
• docs/concepts/embeddings.md — cosine similarity, HNSW, MMR, model selection guide
• docs/api/rag-pipeline.md — RAGConfig + RAGPipeline full parameter tables and examples
• docs/api/state-graph.md — StateGraph, CompiledGraph, TypedState, StateField, checkpointers
• docs/api/agents.md — AgentConfig, ReActAgent, FunctionCallingAgent, AgentExecutor, BaseTool, @tool, HandoffChain, Crew, MCPAgent
• docs/api/memory.md — All 9 memory backends with constructor tables and selection guide
• docs/api/evaluation.md — EvaluationPipeline, 5 metrics, @eval_case, EvalCaseMeta, CLI usage
• docs/api/observability.md — TokenTracer, CostTracker, BudgetGuard, OTelExporter, TracingMiddleware, DistributedTracer

Files modified

• docs/api/retriever.md — Expanded from 1 to 16 retriever strategies (Retriever, HybridSearch, MMR, ContextualCompression, SelfQuery, ParentDocument, MultiQuery, Ensemble, BM25, TimeWeighted, Metadata, LongContextReorder, StepBack, GraphRAG, HyDE, Caching) with selection guide table
• sidebars.ts — Added Concepts category; API Reference expanded from 3 to 9 items

Test plan

• npm run build passes with no errors