Display resolution and rotation sections should be merged

[lurch]

https://github.com/raspberrypi/documentation/blob/bookworm/documentation/asciidoc/computers/configuration/display-resolution.adoc and https://github.com/raspberrypi/documentation/blob/bookworm/documentation/asciidoc/computers/configuration/display-rotation.adoc are fairly similar, so it might be worth merging them to reduce the redundancy? Up to @aallan to decide if he wants to keep them as separate sections or not.

[lurch]

Alasdair has said "Merge them".

[lurch]

And we should probably also change the display-rotation instructions from using wlr-randr (which is only a temporary change, and doesn't survive a reboot) to instead editing config files (which is a persistent change, and doesn't get forgotten about after a reboot).

[lurch]

The rotation-instructions were written in the "immediate but temporary change" style, because that's what the old docs did.

[lurch]

Fixed on bookworm branch.