Document python version policies and other small doc updates.

[rashidakanchwala]

Description

This PR updates the README and Kedro-Viz documentation to include our Python version policies. It also includes a few minor updates to the documentation and README that were made while reviewing these files.

Development notes

QA notes

Checklist

• [ ] Read the contributing guidelines
• [ ] Opened this PR as a 'Draft Pull Request' if it is work-in-progress
• [ ] Updated the documentation to reflect the code changes
• [ ] Added new entries to the RELEASE.md file
• [ ] Added tests to cover my changes