Following up from #14830, this PR adds a new page to the docs for Create Next App. The content is identical to the README created as part of #14830.
Also added a link on the main `getting-started` page to the new docs for Create Next App to help users find more info on Create Next App if needed.
I'm unsure as to whether the content for the documentation needs to be different from the one I wrote for the README. Please let me know if that's the case, and I will be happy to tweak the structure.
