Update syntax guide for sublime-syntax

[FichteFoll]

The current page still covers tmLanguage (YAML version). We can still keep that but it should be deprecated and replaced by a new page covering at least the same topics for sublime-syntax instead. We currently refer to the official syntax def docs, but we could do a better job at explaining more relevant use cases & best practices and only refer to the official docs as the reference.

We should also provide some examples & existing use cases (in the Default packages) for extends since it sees more and more usage.