[JavaFX - Part3] Add introduction in part 3

[laney0808]

Current: Has 3 parts: echoing the user, adding dialog boxes, and adding custom behavior to dialog boxes.

Problem: Lacking an introduction to even handlers in general. Students may lack a general understanding of event handlers, and may face difficulties when using event handlers for different controls.

Proposed: Introduce the general logic for event handlers, some common event handlers for different controls, and a link to more advanced event handlers that are not likely to be used in the Duke project for exploration.

[damithc]

@laney0808 You can send a PR for this.

[laney0808]

fixed by #71