Add better camera doc

[hamishwillee]

This is copied from https://github.com/PX4/px4_user_guide/pull/49 (merged PR that needs more work)

Need to fix up https://docs.px4.io/en/peripherals/camera.html

We need documents in user guide on "how to set up cameras". Once complete, this will be it. I've "seeded" it by copying Camera Trigger tutorial from the devguide.

This will have to cover:

1. Overview of what is covered - integration of still camera for use during missions/survey (and also available for manual triggering). Not video/FPV (link to that if we have docs)
2. Explanation of mechanisms for physically connecting cameras
   • No info in any docs, but "hints" in parameters 1: GPIO 2: Seagull MAP2 (over PWM) 3: MAVLink (forward via MAV_CMD_IMAGE_START_CAPTURE) 4: Generic PWM (IR trigger, servo)
3. Lists of known good/tested /recommended cameras
4. Explanation of how to configure cameras (ie the parameters)
5. Link to other information on using cameras.
6. Links to MAVLink camera API.

Note:

• How to use a camera in a mission would be a separate "mission" based document. We would link to it from here
• The camera mission interface would probably be here.
• POSSIBLY this should also include gimbal integration.

[hamishwillee]

Make sure any info required for Camera Capture driver is added: https://github.com/PX4/Firmware/pull/10295#issuecomment-416093746

[IeiuniumLux]

@hamishwillee, Currently there are two Camera docs online with the same title "Camera Trigger". The one you mention above and another one, but with less content here. Can the latter be retired?

[hamishwillee]

@IeiuniumLux That is news to me - it is not "official". Will see what can be done.