[FEATURE] Rename GETTIING_STARTED to USER_GUIDE

[dblock]

Is your feature request related to a problem?

• GETTING_STARTED is a lot more than a getting started
• We want to ensure all new features are documented
• Other projects have USER_GUIDE.md, seems like it's becoming a convention (example)

What solution would you like?

• Rename https://github.com/opensearch-project/opensearch-py/blob/main/GETTING_STARTED.md to USER_GUIDE.
• Do an editing pass on the guide to fix any small stuff.
• Make sure to link it from README.

[saimedhi]

I am working on this