feat: create a 404 response for API

[Regaez]

The API should return a 404 response for any paths requests with the /api route that don't exist, rather than displaying Grav's HTML 404 page.

For example, https://www.mysite.com/api/some-undefined-endpoint should return something like:

HTTP Status: 404
Content-Type: application/json

{
  "message": "API endpoint not found",
  "documentation": "https://github.com/regaez/grav-plugin-api/tree/master/docs"
}