[kalman] update RV notation

[longye-tian]

Hi @mmcky

This PR updates the RV notation following #898 .

In particular, this extends the new uppercase/lowercase convention to the Implementation section and Exercises:

• use $X_t$ and $Y_t$ for random state and signal variables;
• keep lowercase notation for realizations, density arguments, filter estimates, and code variables;
• clarify that the Exercise 3 plotted errors use simulated realizations.

Best,
Longye