docs(how-to/file-route-conventions): add "Catch-all Route" section

[davidbielik]

followup to #13764 to document 404 with file routing

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[timdorr]

Thanks!

[davidbielik]

@timdorr do you know how to set this route to always return 404 status code? Would that just be done in its loader I assume?

export const loader = async ({ request }: Route.LoaderArgs) => {
  return data({}, 404);
};