doc: board_porting: Move the board extensions section

It was awkwardly placed in the middle of the "Write your devicetree"
section, breaking up the flow of the guide. A more natural placement
would be underneath "Contributing your board", considering that this
feature deals with boards already submitted to Zephyr.

Let's relocate this part and swap two of its paragraphs in the process,
to motivate the feature before describing it.

Signed-off-by: Grzegorz Swiderski <grzegorz.swiderski@nordicsemi.no>