flesh out remaining sections from the docs notebook

[sritchie]

;; #### Transformations and constraints

;; When wrapping a Movable Point in a mafs/Transform, the point will be
;; transformed too. However, your constrain function will be passed the
;; untransformed point, and its return value will be transformed back into the
;; currently applied transform. In other words, Mafs takes care of the math for
;; you.

;; Let's see a more complex example where we combine more interesting constraint
;; functions with transforms. On the left, we have a point that can only move in
;; whole-number increments within a square, and on the right, a point that can
;; only move in $\pi / 16$ increments in a circle.

;; TODO demo

;; #### Dynamic Movable Points

;; useMovablePoint is a hook that helps you instantiate and manage the state of
;; a MovablePoint. However, if need be, you can also use MovablePoint directly.
;; This can be useful if you need to work with a dynamic number of movable
;; points (since the React "rules of hooks" ban you from dynamically calling
;; hooks).

;; TODO

;; ### Accessing transforms in JavaScript

;; Here's an example of a custom component that uses the hooks approach to
;; render a grid of points. Because we want the grid's points to have a radius
;; of 3 pixels (regardless of the viewport or any transforms), we use the
;; mafs/useTransformContext hook to get the user and view transforms and apply
;; them to the circles' x and y coordinates, but not to their radius (which
;; is in pixels). We also cannot use the CSS transforms approach here, because
;; that would distort each circle.

;; TODO demo

;; ## Animation

;; useStopwatch allows you to start and stop a real-time clock for doing neat
;; things like physics simulations.

;; Pass :startTime (defaults to 0) or :endTime (defaults to Infinity) to
;; constrain the stopwatch.

;; TODO not working yet!

;; ## Examples

;; ### Bézier curves
;; ### Riemann sums
;; ### Fancy parabola
;; ### Projectile motion