Move guides out of "API Reference" #214

I'd argue that the Reference should be reserved for, well, terse but complete reference materials, like https://developers.deepl.com/api-reference/translate/request-translation . I'd place most or all "Overview" sections currently in our Reference in the Documentation tab or such. And I'd definitely move out things like this glossary guide, this guide to v2 vs v3 glossary endpoints, this guide to managing admin API keys, etc.