move docs images to docs/images
update network docs
Add SpriteManager

Currently its sole point is to draw quicker by making it so drawPrep is only
called once for all sprites.

Ideally it should also handler drawing order, maybe hierarchy. Maybe I'll make
a separate one for that.
Update to use SpriteManager,

Also limit max number of local players to max that can fit on scale 1 map.

Note: The game will still crash if you resize the canvas it starts if the new
size doesn't have enough space for all the players.

This is because the number of players is normally determined by how many users
connect to the game. I suppose I could fix it so AI players are added and
deleted as needed but AI players were just a hack to see lots of players
on the screen.