DoodleBUGS: SSM plates notation

[yebai]

The plate notation can be generalised to specify state-space models (SSMs):

Each SSM is fully characterised by the prior over the initial state, $p(x_0)$; the transition dynamics, $p(x_t \mid x_{t-1})$; and the observation model, $p(y_t \mid x_t)$. All of these distributions can be viewed as parameters of the SSM plates, and specified in the "Props" panel.

@FredericWantiez @charlesknipp