Data::Rx::Manual::CustomTypes pod and example/

"This documentation covers 3 worked examples showing different
facets on creating a type plugin.

The examples themselves are runnable, within the example/ directory
and come with a test.pl to demonstrate their use.

There are also some minor tweaks to the main Data::Rx documentation,
mostly to point at this new POD doc."

Unsure about a few bits for this contribution:

 * I've not added the dependencies for examples/ to dist.ini (don't really know dzil, is there an ExamplesRequires plugin?)
 * do I need to do anything with pod-weaving to add the additional section to Data::Rx itself?
 * is the main Data::Rx::Manual::CustomTypes ok as a .pod like so, or does it require other config for the pod-weaving setup?

Shout if you'd like me to rework anything!