Add e2e test for publishing a page

[oandregal]

Fix #9087.

[tofumatt]

I made some documentation tweaks but looks good. 👍 We might want to check that other behaviour for pages are covered as well, but this is a great start!

[tofumatt]

This doesn't really feel like a test related to the publishing of a post in the first place, but I get it was like that when you got here 😄

[aduth]

FWIW we could avoid code duplication by iterating types if all other logic is basically the same:

[ 'post', 'page' ].forEach( ( postType ) => {
	describe( postType, () => {
		beforeEach( async () => {
			await newPost( postType );
		} );

		// ...
	} );
} );