Use literalinclude for embeded code examples in documentation

We have code examples for documentation stored in `examples/tutorials/doc_examples`, but then the content tends to be copied and pasted in the source.
e.g. In https://github.com/enthought/traits/blob/1af120c7d68356105d66ec6458967b5660e37e3f/docs/source/traits_user_manual/advanced.rst

Contains the same content as the code in https://github.com/enthought/traits/blob/44ca101bcfa2e5a3c31d5fab141e892fd767793d/examples/tutorials/doc_examples/examples/override_default.py

We can use `literalinclude` in the source to remove this duplication.