Add initial docs for Entity Pathfinders and Mob Goal API

[olijeffers0n]

I have not extensively used either of these API's in the past, however these seem to work

closes #265

[MiniDigger]

this needs notes on how vanilla selects which goals to activate and prolly some more caveats on how pathfinding works from somebody who actually uses the pathfinder, iirc it can ge all kinda wonky to call moveTo every tick.