[heycam/webidl] Add an intro section to WebIDL for newcomers to the web platform (#918)

Newcomers to the web platform often encounter WebIDL-specific terms like "interface" and "mixin", and they tend to map baggage from other object-oriented systems onto those terms, which can cause problems when trying to carry across the fact that an "interface" is more like a Java/JS class. We should add a section that clearly specifies the meaning of these terms and the role of WebIDL specifying a boundary between the browser internals and JS-land.

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https://github.com/heycam/webidl/issues/918