feat: update README file for gamexblock

[viv-helix]

Description

This PR updates the README.md for the edx-games XBlock to improve clarity and documentation quality.

Changes

• Added a clear overview of the Games XBlock and supported game types.
• Documented features including image support, shuffling, timer, pagination, confetti animation, and encrypted answer keys.
• Added requirements and dependency versions.
• Included installation instructions via pip.
• Added step-by-step usage instructions for enabling the XBlock in Studio.
• Documented configuration options, including: Waffle flag setup, Custom storage backend (filesystem and S3 examples)
• Added a security section describing answer-key encryption, JS obfuscation, and upload validation.

Overall, this improves developer and platform operator guidance for installing and configuring the edx-games XBlock.

Jira

• https://2u-internal.atlassian.net/browse/TNL2-164