docs: update gpt index references to LlamaIndex

[DeadBranches]

The GPT Index project is transitioning to the new project name, LlamaIndex.

I've updated a few files referencing the old project name and repository URL to the current ones.

From the LlamaIndex repo:

NOTE: We are rebranding GPT Index as LlamaIndex! We will carry out this transition gradually.

2/25/2023: By default, our docs/notebooks/instructions now reference "LlamaIndex" instead of "GPT Index".

2/19/2023: By default, our docs/notebooks/instructions now use the llama-index package. However the gpt-index package still exists as a duplicate!

2/16/2023: We have a duplicate llama-index pip package. Simply replace all imports of gpt_index with llama_index if you choose to pip install llama-index.

I'm not associated with LlamaIndex in any way. I just noticed the discrepancy when studying the lanchain documentation.