Add "New Page" option for 404

[sorat0mo]

Improvement on #1004

This should work, but it cannot distinguish new pages from new posts, i.e. When you try "abc.github.io/2022-07-14-abcde" it will just direct you to create a file called "2022-07-14-abcde" in root.

Example when the page does not exist (Notice the text in red boxes)

[daattali]

Thanks @sorat0mo , I appreciate the effort. But for the sake of not bloating the codebase with too many esoteric features, I prefer not to merge this feature. "Edit this page" is something that can be used by anyone who goes to your website when they see an issue and they want to help you fix it; "New page" button is only for yourself and it sounds like a bit of a strange use case. I prefer not to add code/features that are not going to be very popular.