Add manual tuning instructions to SysId introduction

[calcmogul]

• Ks = the voltage that makes the robot start moving

This isn't quite accurate because dynamic friction is involved, but it's a starting point.

• Kv = (12 V - Ks)/max speed

The better way to find Ks and Kv is to do a steady-state voltage-velocity line fit, which gives Kv as the slope and Ks as the y-intercept (voltage = Ks + Kv * velocity).

• Ka = whatever makes the robot follow a trapezoid profile well with feedforward-only

This gain can be annoying to tune for some robots. It can be set to zero unless the robot mechanism requires a lot of voltage to accelerate.