docs: move 'Linking between pages' earlier in layouts-and-pages
The reader learns to create a page and a layout, then immediately needs to
know how to navigate between them. The previous flow buried Link after
nested routes, dynamic segments and search params, so the introductory
example for Link sat hundreds of lines below where it would first be
useful.
Reorder the section after 'Creating a layout' and before nested routing.
Picked up from #82773.
