Extract code snippets from structure-of-a-presentationml-document.md

Extract the code snippets from [docs/presentation/structure-of-a-presentationml-document.md](https://github.com/OfficeDev/open-xml-docs/blob/main/docs/presentation/structure-of-a-presentationml-document.md) and use docfx snippets to retrieve them from the samples folder and audit the text.