[Suggestion] Add position controller details, including tuning instructions

[ES-Alexander]

Basis

Position control is generally important (and often critical) to most ArduPilot vehicles, either in full or partial degrees (e.g. altitude hold is vertical position control). It seems valuable to include information about the relevant parameters, ideally including how to go about tuning them.

Context

Raising in response to comments in this PR:

@ES-Alexander: Is the intent for PSC params to be covered elsewhere/later? They're relevant for depth and position control and the like.

@Hwurzburg: those are considered advanced...and we have no pages in any vehicle wiki on how to tune them or even how to know you need to tune them.....certainly could be added but would need Leonards input I would imagine

Relevant Sources

• @rmackay9 has previously provided some insight in a discussion about Rover S-curves
• @amilcarlucas has seemingly included some options (for various vehicle types) in the ArduPilot Methodic Configurator
  • I'm unsure whether this actually walks through how to tune them, or just provides a space to do so
• Chris Rosser's ArduPilot tuning guide includes throttle (vertical) controller and horizontal controller tuning processes for ArduCopter
  • Without a posted license, I'm unsure whether there are issues with including/referencing such content in an open source wiki 🤷‍♂

[rmackay9]

@lthall probably has the best advice on this and any advice would probably be a new page linked from here or maybe here.

My understanding is that most users should not need to adjust the PSC_xxx tuning values but if they do, they normally need to be lowered by up to 1/2 their original value. I do not know the process for determining the best values though