add smoothing to BlazePose under tfjs

[ziyuan-linn]

Opening an issue to track the problem discovered in #140. enableSmoothing: true does not seem to work for BlazePose under tfjs runtime.

[shiffman]

In the meantime, we can add something to the documentation (cc @QuinnHe) about how mediapipe can be used instead of tf.s in order to enable smoothing. This likely does not need to be corrected since we have that as a backup option and perhaps we will eventually return the default to media pipe after resolving #69 and #129!

[QuinnHe]

Noted! Will work on it after other pages are completed :)