Extending Swift-DocC to support Objective-C documentation

[Kyle-Ye (Kyle)]

@philipturner

I have had enough frustration with Apple making their Metal sample code in Objective-C (even code released in 2021), and if Apple made new Metal tutorials using DocC, I'd like them to stick to exclusively using Swift.

Metal was introduced in 2014 the same year with Swift. Since Swift was a baby at that time. Metal was of course built upon with ObjC. Even we can use swift to get its API, but its core is still ObjC oriented.