Documentation: Clarify declaration files and the declare keyword

TypeScript Version: 2.8.0

Search Terms:
document declaration files

** Issue**
It doesn't look like you actually define what a declaration file is in your documentation. For instance, your introduction to these at https://www.typescriptlang.org/docs/handbook/declaration-files/introduction.html tells me you're going to help me learn how to write them, but not what they are or why I'd want one. But, I don't think this makes it easy for a newbie to understand.

Similarly, you don't seem to explain when the declare keyword is, or when it should be used (as far as I can tell, it is used when you need to tell the compiler that the module/library/whatever has a value of some type).

You also seem to make some references to the global namespace (declare global { ... })), but don't seem to directly explain it anywhere.

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Beginne mit der Einführung in die Declaration Files unter https://www.typescriptlang.org/docs/handbook/declaration-files/introduction.html und verfolge anschließend deren Verweise auf declare global und verwandte Anleitungen zu Declaration Files. Prüfe das umgebende Handbook-Material, bevor du Änderungen vornimmst. Erledigt ist die Aufgabe, wenn die Dokumentation Declaration Files klar definiert, erklärt, warum und wann declare verwendet werden sollte, und den globalen Namespace direkt erläutert.