Improves documentation, and adding DocC (#6784)

Finished documenting flatbuffersbuilder

Replaces swift 5.5 with 5.2 packages

Starts building the tutorial for xcode 13

Finishes building the tutorial for xcode 13

Removes docc files from old swift versions

Updates swift style guide

swift/Sources/FlatBuffers/FlatBuffersUtils.swift

@@ -29,6 +29,8 @@ public enum FlatBuffersUtils {
   /// creates a new buffer use `readPrefixedSizeCheckedRoot` instead
   /// unless a completely new buffer is required
   /// - Parameter bb: Flatbuffer object
+  ///
+  ///
   public static func removeSizePrefix(bb: ByteBuffer) -> ByteBuffer {
     bb.duplicate(removing: MemoryLayout<Int32>.size)
   }