docs: add Haystack + Valkey cookbook

[edlng]

Summary

Adds a two-part Haystack + Valkey cookbook with a deterministic Python sample. The walkthroughs cover ValkeyDocumentStore, vector retrieval, metadata filters, and a RAG pipeline with optional model-backed embedding and generation paths.

Issue

N/A. This follows the cookbook acceptance criteria in PR #40.

Changes

• README.md and cookbook index files: add navigation for the new Haystack integration.
• cookbooks/framework-integrations/haystack/: add getting-started and RAG walkthroughs with prerequisites, security guidance, configuration references, teardown steps, and navigation.
• cookbooks/framework-integrations/haystack/sample/: add a runnable deterministic sample using fixed four-dimensional embeddings, Haystack's document store and retriever APIs, metadata filtering, environment overrides, and cleanup tests.
• sample/requirements.txt: pin valkey-glide and valkey-glide-sync to 2.5.0, alongside the Haystack integration and test dependencies.
• haystack/README.md: disclose that Haystack is maintained by deepset.
• .github/workflows/cookbook-python.yml: add temporary Python lint and integration-test coverage for Haystack against Valkey Bundle 8.1.4 and 9.1.1, with SHA-pinned actions.

Tests

• VALKEY_PORT=6381 .venv/bin/python -m pytest -q against Valkey Bundle 8.1.4: 4 passed.
• VALKEY_PORT=6382 .venv/bin/python -m pytest -q against Valkey Bundle 9.1.1: 4 passed.
• ruff==0.11.13, Python compileall, scoped Markdownlint, Actionlint, Lychee, and git diff --cached --check passed.
• E2E test with the sample file.

Blockers / Future work

• Keep the temporary Python workflow until PR docs+ci: add semantic-caching cookbook with Python CI (MERGE FIRST FOR PYTHON CI) #45's shared workflow merges, then remove this copy and add Haystack to the shared matrix.
• The default sample path is credential-free and uses fixed vectors. The optional SentenceTransformers and OpenAI paths require their own packages, model downloads, or credentials.