Support GitHub's "Discussions" in GitHub site module

[PeterDaveHello]

Is your feature request related to a problem? Please describe.
新功能是否与解决某个问题相关, 请描述

The ChatGPTBox extension currently supports parsing content from GitHub issues and pull requests, but not from GitHub Discussions.

Describe the solution you'd like
你期望的新功能实现方案

Support parsing a GitHub Discussion page just like GitHub Issues and Pull Requests

Additional context
其他

GitHub Discussions is a dedicated space for conversations and general discussions within a repository. Supporting it would provide more comprehensive GitHub integration in the ChatGPTBox extension. Though the GitHub Discussions feature is not enabled in this project, but we can take a look at the docs and other projects for it:

• https://docs.github.com/en/discussions/quickstart
• https://github.com/markedjs/marked/discussions