Update requirements documentation.

[ayushdg]

Description

Currently NeMo Curator is only tested with Ubuntu & does not work with windows like it's come up in #76. This PR aims to update the documentation to reflect OS requirements and GPU requirements.

Usage

N/A

Checklist

• [X] I am familiar with the Contributing Guide.
• [X] New or Existing tests cover these changes.
• [X] The documentation is up to date with these changes.