Fix minor README docs issues in client reference

[austin-denoble]

Problem

@haruska pointed out that the headers in the SDK client reference navigation bar were a little off for "Create Index" showing "Create a pod index" only. The formatting also felt a bit off with everything being aligned.

https://sdk.pinecone.io/python/pinecone.html

While fixing this I also noticed the link for the CONTRIBUTING.md file is relative in the README, which doesn't work when linking from our generated docs.

Solution

• Promote "Indexes" to a top-level section to visually break up the nav list a bit, and then swap Create Index to the header and moving Create a serverless index and Create a pod index to sub-headers of that.
• Link directly to github for the CONTRIBUTING.md file.

Type of Change

• Non-code change (docs, etc)

Test Plan

Before	After

You can pull this branch down and build the docs locally. You'll need to make sure to install the grpc extras with poetry:

poetry install --extras "grpc"
poetry run pdoc pinecone '!pinecone.core' '!pinecone.utils' --favicon ./favicon-32x32.png --docformat google -o ./docs

This will build the docs in the /docs folder under the project folder. You can open the index.html file locally.

[haruska]

Thanks for cleaning this up!