Updates for synchronous and externally triggered cameras

[njhollinghurst]

Update synchronized cameras documentation to:

• Deprecate the XVS sync method for GS Cam, in favour of XTRIG

• Recommend "always-on" property instead of using pull-ups

• Recommend "trigger-mode" for IMX477 be set using the new OF property (better for source and sink on same Pi) while noting that the module parameter still works and that "trigger-mode" means something quite different for IMX296!

• Document the need to edit the pipeline config file for timeout

• Some attempt to tidy the structure while covering all the options

• Remove $ prompts from scripts/commands, so they can be copy-pasted

[lurch]

• Remove $ prompts from scripts/commands, so they can be copy-pasted

If you use the [source,console] above a code-block, then the $ is automatically non-copyable, see e.g. https://www.raspberrypi.com/documentation/computers/camera_software.html#rpicam-jpeg

[njhollinghurst]

It's a bit messy, but hard to make it simpler without omitting required information (indeed it does omit repeating the "timeout-defeat" instructions for GS external trigger, but presumably people will infer that it needs to be done as in the previous section).

I feel this stuff would be better as a separate Application Note rather than part of the generic camera documentation?

Conspicuous by is absence is any mention of the new S/W synchronization method!

[njhollinghurst]

Ah, I've belatedly noticed that we do already document Software Camera Synchronisation, although it's well hidden under rpicam-apps and could maybe do with being bumped a bit up the hierarchy.

I should definitely link there from here.

[naushir]

LGTM