docs: Add Phoenix Llamaindex RAG Demo notebook + chunks + questions

[saikolasani]

[review-notebook-app[bot]]

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[mikeldking]

nit: can we rename to demo_llama_index just to keep track of demo data creation notebooks?

[cjunkin]

resolves #4240

[cjunkin]

Cohesive use case questions fixture: https://console.cloud.google.com/storage/browser/arize-phoenix-assets/demos?pageState=(%22StorageObjectListTable%22:(%22f%22:%22%255B%255D%22))&project=phoenix-202055

RAG demo traces and evals (demo_llama_index_rag_): https://console.cloud.google.com/storage/browser/arize-phoenix-assets/traces?pageState=(%22StorageObjectListTable%22:(%22f%22:%22%255B%255D%22))&project=phoenix-202055

[cjunkin]

Demo traces available on hosted phoenix

[cjunkin]

However, seems like document relevance eval fixtures have some formatting problems. This isn't an issue with the code, however, and seems to be how the doc relevance eval spans were generated. Seems like rerunning the demo and uploading new fixtures to GCS would fix the issue. Merging PR