"New page" button shouldn't show for collections that it doesn't work for

[kaganjd]

Is your feature request related to a problem? Please describe.
In the Netlify forum, someone was trying to use "New page" on a file collection. Clicking the button returned a blank page. This is by design, not an error, but maybe we could make this more intuitive for people.

Describe the solution you'd like
Don't show "New page" button on collection types it won't work for.

Additional context
https://community.netlify.com/t/click-new-page-shows-empty-page/21337/5

[erezrokah]

Thanks @kaganjd, since the create: true option is opt in for folder collections, we could probably prevent users setting it for file collections during schema validation.