Document how to create a new converter (or extend an existing one)

[mojavelinux]

Although covered in the API docs, the formal documentation does not explain how to create a new converter or extend an existing one (including built-in converters).

We could consider putting this new page under Extensions. Though it might be better to add the page under a new category named Converters, and moving the Available Converters page under it too.

Here's the existing information on the topic: https://github.com/asciidoctor/asciidoctor/blob/master/lib/asciidoctor/converter.rb#L4-L43

[mojavelinux]

For reference, both AsciidoctorJ and Asciidoctor.js have a page on how to make a custom converter for their respective platforms.

• https://docs.asciidoctor.org/asciidoctorj/latest/write-converter/
• https://docs.asciidoctor.org/asciidoctor.js/latest/extend/converter/custom-converter/

[mojavelinux]

This page should list all the contexts (both block and inline) that the base converter maps to a handler method and the name of the method. It should then explain that you can override the convert method to customize this context to method name mapping.

[mojavelinux]

Backport this change to 2.0.x.