Add a glossary for terms like "clip behavior"

[jackieoh1]

Page URL: https://flutter.dev/docs/release/breaking-changes/clip-behavior.html
Page source: https://github.com/flutter/website/tree/master/src/docs/release/breaking-changes/clip-behavior.md

Description of issue: can you imagine being relatively new to flutter, searching for what a clip IS, seeing "clip behavior" as a flutter documentation topic, with the entire page not bothering to describe what clip behavior is?

your documentation is beyond frustrating for a newer programmer.

[sfshaza2]

Hmmm. Yes, finding yourself on a breaking change page is not going to be helpful. The breaking change pages are intended for folks who are familiar with Flutter, and have written code using Flutter, and need to migrate their app to continue working, as the API has changed. They aren't intended to teach basics.

This query suggests to me that perhaps we need a glossary page. Would that be helpful?