Docs improvement

[wmurphyrd]

Snippet from slack

<!-- Camera -->
<a-entity   id="camera"
            position="0 0 0"
            rotation="0 0 0"
            camera="far: 500; userHeight: 1.6"
            kinematic-body="radius: 0.7"
            universal-controls="enabled: true; fly: false; movementAcceleration: 80; movementControls: touch, keyboard; movementEasing: 15; movementEnabled: true; movementSpeed: 5; rotationControls: hmd, mouse; rotationEnabled: true; rotationSensitivity: 0.05">
            <a-entity id="player" physics-collider collision-filter="collisionForces: false"></a-entity>
</a-entity>

<!-- Trigger Collider -->
<a-entity   id="platform"
            position="0 1.5 -10"
            rotation="0 0 0"
            static-body
            physics-collider
            geometry="height:3; width:3; depth:3"
            material="visible: true"
            collision-filter="collisionForces: false; collidesWith: player">
</a-entity>

• Explain collision groups (not related to HTML id/class)
• Explain collision group defaults and impact on bodies that don't have the collision-filter component
• Collider requires a body