Update to svelte/easing documentation

[chasec22]

Describe the problem

The documentation for Svelte Easing Documentation isn't the most straightforward. It shows off all of the involved functions, but no text describes what you're looking at. Not the biggest deal, but it make sense to introduce it seeing as the other references include descriptions.

Describe the proposed solution

A simple description would probably do. I don't think you would need one for each of the functions, maybe just an overarching description such as "Easings are functions that can be used for different animation styles..." similar to what exists under svelte/action or svelte/motion.

Importance

nice to have