fix: correct getting-started friction in zcookbook quickstarts

[mervivian]

##summary

• Fix cd zeroentropy-cookbook → cd zcookbook in root README getting-started block (the cloned directory is named zcookbook, not zeroentropy-cookbook)
• Create guides/index_and_query_quickstart/data/.gitkeep so the data/ directory exists; index.py calls os.listdir("./data") and crashes with FileNotFoundError if it isn't present
• Fix folder name in index_and_query_quickstart/README.md file-structure diagram: index_and_query/ → index_and_query_quickstart/
• Fix minimum Python version in reranker_quickstart/README.md: 3.8+ → 3.9+ to match the zeroentropy SDK's own requires-python = ">=3.9"

##How to reproduce

1. Follow the root README "Getting Started" section verbatim — the cd command fails immediately.
2. cd guides/index_and_query_quickstart && python index.py — crashes with FileNotFoundError: [Errno 2] No such file or directory: './data'.
3. reranker_quickstart README advertises Python 3.8 support, but pip install zeroentropy on Python 3.8 fails because the SDK requires 3.9+.

##Test plan

• Clone a fresh copy of the repo and run cd zcookbook — succeeds
• cd guides/index_and_query_quickstart && ls data/ — directory exists
• python index.py with no files in data/ returns "Indexing 0 documents" instead of a FileNotFoundError
• reranker_quickstart/README.md now accurately reflects SDK requirements