Split out guide about predicates

[1ec5]

MGLVectorStyleLayer.predicate has documentation with enough detail that it could stand on its own as a jazzy guide, similar to the one for tile URL templates. We already use predicates in multiple parts of the API, including the feature querying methods of MGLMapView and MGLVectorSource, so centralizing the documentation in a guide would make it easier to see all those APIs as being related. Once we replace MGLStyleValue with NSExpression (#8074), it’ll be all the more important to document expressions and predicates in one place.

This documentation doesn’t differ at all between iOS and macOS, so all we need is a Markdown document in platform/darwin/docs/guides/. However, we’ll also need a small amount of code generation if we introduce any code examples into that guide.

The new guide should link to this section of the “Information for Style Authors” guide that discusses predicates. I think that section can stay where it is, because folks who only work with the runtime styling API won’t need to see it.

/cc @captainbarbosa @jmkiley @anandthakker