docs: update gpt index references to LlamaIndex (#1856)

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](https://github.com/jerryjliu/llama_index):
> 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.

docs/modules/indexes.rst

@@ -4,7 +4,7 @@ Indexes
 Indexes refer to ways to structure documents so that LLMs can best interact with them.
 This module contains utility functions for working with documents, different types of indexes, and then examples for using those indexes in chains.
 LangChain provides common indices for working with data (most prominently support for vector databases).
-For more complicated index structures, it is worth checking out `GPTIndex <https://gpt-index.readthedocs.io/en/latest/index.html>`_.
+For more complicated index structures, it is worth checking out `LlamaIndex <https://gpt-index.readthedocs.io/en/latest/index.html>`_.
 
 The following sections of documentation are provided: