Document extensions

[jasonkuhrt]

Perceived Problem

There is a concept of extensions, which is the ability in the builder to add custom api surface area to the controller.

We actually use this feature in our examples: https://github.com/jasonkuhrt/alge/blob/main/examples/Moniker.ts#L45-L62

This is not documented.

Ideas / Proposed Solution(s)

Document it in our features. This is probably a new category, either before or after codecs, but lower anyways, since it is perhaps more advanced.

[jasonkuhrt]

Also mention that extensions are available to decoder context.