Automatically generate 404.html if a custom 404 view exists

[jesseleite]

Right now to get a custom 404 page on netlify we had to...

1. Create a _redirects file in your project that references a fake 404 page (see docs)...

   /* /not-found 404
   

2. Create a custom laravel 404 view at resources/views/errors/404.antlers.html
3. Map a route for it Route::statamic('not-found', 'errors/404')
4. Add /not-found to urls in your ssg config
5. Add base_path('_redirects') => '_redirects' to copy in your ssg config

It looks like netlify will use 404.html if it exists, so maybe we could automatically detect if a custom 404 view exists and generate that to 404.html.