docs: add 9 how-to guides

[AmitoVrito]

Summary

• Implements issues How-to guide — Streaming responses (token-level, SSE, WebSocket) #56–How-to guide — Migrate from LangChain to SynapseKit #64: 9 task-focused how-to guide pages in docs/how-to/
• Adds How-to Guides category to sidebars.ts (after Concepts, before RAG)
• Each guide is 150–250+ lines with complete, runnable code examples and expected output comments

Guides added

File	Issue	Description
docs/how-to/streaming.md	#56	Token-level, SSE, WebSocket streaming (8 patterns)
docs/how-to/custom-llm-provider.md	#57	Extend BaseLLM, streaming, tool calling, MockLLM
docs/how-to/custom-tools.md	#58	@tool decorator, async tools, BaseTool subclass
docs/how-to/custom-retrievers.md	#59	SQL, Elasticsearch, EnsembleRetriever, caching
docs/how-to/memory.md	#60	ConversationMemory, SQLite, Redis, shared memory
docs/how-to/error-handling.md	#61	Retries, FallbackLLM, BudgetGuard, GraphInterrupt
docs/how-to/testing.md	#62	MockLLM unit tests, @eval_case, GitHub Actions CI
docs/how-to/production.md	#63	Dockerfile, docker-compose, gunicorn, OTel, CI/CD
docs/how-to/migrate-from-langchain.md	#64	Side-by-side LangChain vs SynapseKit comparison

Test plan

• npm run build passes (webpack compiles client + server successfully)
• All 10 files present in docs/how-to/
• sidebars.ts updated with single How-to Guides category

Closes #56 #57 #58 #59 #60 #61 #62 #63 #64