[ENHANCE] English version guideline

[PKUFlyingPig]

Now the website already supports the English version and all the old materials have been translated into English. If you want to add a new course, please follow the guidelines below to provide both the Chinese and English versions.

• Fork the repo and clone your own copy
• Add your Chinese course description in docs/your_topic/course_id.md
• Create course_id.en.md under the same directory of course_id.md (course_id must be same)
• Translate your Chinese description into English and fill it into course_id.en.md. You can adjust some sentences to read more fluently and natively. Some online translation tools may help you, like DeepL and Google Translation.
• Create a Pull Request for review. If everything looks good, it will be merged into the main branch.

[PKUFlyingPig]

#544