docs: Add pageExtensions usage note in API Routes

[menosprezzi]

In order to turn the API Routes docs more clear about the file extensions when using pageExtensions.
I know that it was already explained here.

But just think about the DX: If a developer have a trouble with an API endpoint not working properly should it check it on custom-page-extensions page or api-routes page itself??

Related to #8178.

Bug

• Related issues linked using fixes #number
• 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