Move metaclass documentation to appear later in the docs (#5216)

Metaclasses are a pretty specialized topic so moving them a bit
further towards the end of the documentation.

Author: JukkaL

docs/source/index.rst

@@ -32,14 +32,14 @@ Mypy is a static type checker for Python 3 and Python 2.7.
    kinds_of_types
    class_basics
    protocols
-   metaclasses
    python2
    dynamic_typing
    casts
    duck_type_compatibility
    stubs
    generics
    more_types
+   metaclasses
 
 .. toctree::
    :maxdepth: 2