Document how to write a `transform` plugin

[choldgraf]

We have a nice plugins guide in the documentation and it mentions the ability to write Transforms as plugins, but there's no example for how to do it.

The transforms in the mystmd package seem to be written in TypeScript rather than the .mjs extension that is used for the plugin example. They also seem to do a lot of importing of other npm packages.

It would be helpful to have a short guide that describes how to write a basic transform, along with a short template that others can start from.