change documents mission planner save folder

#### Issue details
C:\Users\username\Documents\Mission Planner
How change default mission planner save folder?

#### Version
_What version was the issue encountered with_

#### Platform
[ x] All
[ ] AntennaTracker
[ ] Copter
[ ] Plane
[ ] Rover
[ ] Sub

#### Airframe type
_What type of airframe (flying wing, glider, hex, Y6, octa etc)_

#### Hardware type
_What autopilot hardware was used? (pixhawk, pixracer, PX4FMU etc)_

#### Logs
_Please provide a link to any relevant logs that show the issue_

The issue only identifies the current folder as C:\Users\username\Documents\Mission Planner and asks how to change it; no source file or test is named. Start by searching the Mission Planner C# code and configuration handling for that default path. Done means documenting an existing way to select a different save folder, or identifying that no such option exists.

Written by the indexing model from the issue text.