Update details about Cache Control Headers

[MaedahBatool]

We need to update details about adding Serverless Functions' Cache-Control headers to next.config.js files. And inform developers that these headers cannot be set in next.config.js files.

Bug

• Slack Thread
• Integration tests added
• Errors have helpful link attached, see contributing.md

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.
• Errors have helpful link attached, see contributing.md

Documentation / Examples

• Make sure the linting passes by running yarn lint