ArduSub 4.1 transfer

[ES-Alexander]

• high level "what's new from 4.0", (briefly) covering
  • EKF3
  • Visual Odometry
  • our 4.1 release notes
• supported sensor types (incl. relevant parameters, MAVLink messages, calibration, and code library links)
  • accelerometer
  • barometer (distinguish between wet and dry)
  • gyroscope
  • magnetometer/compass
  • temperature
  • leak
  • positioning (cover GPS (serial and MAVLink), DVL (visual odometry))
  • power sensor
  • rangefinder/altimeter (e.g. Ping Sonar)
  • other (search MAVLink?)
• safety/failsafes (incl. power management)
• supported output types (incl. relevant parameters and MAVLink messages)
  • thrusters
  • servo-style PWM controls (lights, motors/servos (e.g. camera gimbal), gripper, etc)
  • general PWM controls (requires direct MAVLink, can use duty cycle, and can do timed cycles between specified and trim)
  • digital outputs ("relays")
  • MAVLink camera gimbals
• transfer stuff
  • build docs
  • features in operation (incl. joystick button functions)
  • custom frames
  • data logging
  • SITL docs
  • RC input and output
  • parameter list
    • fetch from https://github.com/ArduPilot/ParameterRepository
    • review options hidden by parser script (e.g. CIRCLE and WPNAV should perhaps be shown)
    • relevant to ArduSub: document used parameters #29
  • applications? (unsure if this fits here anymore)
• link to MAVLink and general ArduPilot docs
• list all handled/outputted MAVLink messages
  • see Tools: scripts: create mavlink_parse.py ArduPilot/ardupilot#22278
  • relevant to ArduSub: include supported MAVLink messages #20

Longer term:

• improve parameter list descriptions, and remove/hide unused parameters
• check all MAVLink messages for full and correct support
• improve build and custom frame docs
• detailed breakdown/walkthrough of ArduSub codebase