Add docs regarding the environment vitepress used

[gweesin]

Is your feature request related to a problem? Please describe.

about #3851

Describe the solution you'd like

The usage of environment variables, such as process.env.VITE_EXTRA_EXTENSIONS, warrants a more detailed description in the official documentation.

If I haven't seen the source code, I wouldn't know that VITE_EXTRA_EXTENSIONS is a string that concatenates different words with commas.

Describe alternatives you've considered

No response

Additional context

No response

Validations

• Follow our Code of Conduct
• Read the docs.
• Read the Contributing Guidelines.
• Check that there isn't already an issue that asks for the same feature to avoid creating a duplicate.