Touch Display installation needs better instructions

[lurch]

The photo at https://www.raspberrypi.com/documentation/accessories/display.html#physical-installation doesn't seem very clear as to exactly which pins are being connected to which. Especially given that the pin-labels on the Touch Display's PCB aren't readable once a Raspberry Pi has been screwed onto the standoffs!

We should also add some wording about "if the touchscreen printed circuit board is powered directly", similar to that shown in https://datasheets.raspberrypi.com/display/7-inch-display-product-brief.pdf

[aallan]

See also #2977

[aallan]

So #3121 and subsequent changes by me to fix the narrative have fixed some of the problems here, but it needs an actual installation walkthrough, with step-by-step pictures.

Possibly we should use GIFs like the Build HAT instructions for this. That'll need Brian or possibly Jack involved.

[aallan]

One for you @nathan-contino, but networking things first!

[nathan-contino]

The installation instructions have since been overhauled significantly. I think this has been addressed!