Update environment-variables.md

[PaulvdDool]

I lost quite a few hours figuring out why my environment variable was undefined in the browser.
I read about the built-in support and was like: "Oh nice" and didn't read much further. I missed the part about how you need to prefix your variables with NEXT_PUBLIC_ in order to expose them to the browser.
I think a hint to this in the anchor link to that section will make it more discoverable as it's then mentioned near the top and will save people who are like me some time and a headache.

Bug

• Related issues linked using fixes #number
• Integration tests added

Feature

• Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
• Related issues linked using fixes #number
• Integration tests added
• Documentation added
• Telemetry added. In case of a feature if it's used or not.

Documentation / Examples

• Make sure the linting passes