Docs: Updated Custom Filters (#395)

* Docs: Updated Custom Filters

Updates the custom filter documentation to the latest API changes.

This also includes using mdbook's `include` preprocessor to inject the
example code into the documentation.

Note: Because of the use of `include` preprocessor I had to remove the
Markdown lists, as they wouldn't format correctly
(See: rust-lang/mdBook#1564).

Closes #373

* Fix CI build
* Review Updates.