Add `atomworks` to extension package page

[padix-key]

Recently https://github.com/RosettaCommons/atomworks was released, which uses AtomArray and AtomArrayStack as structure representation. Hence, I think it would be fitting to include it in the list of extension packages.

@Croydon-Brixton @nscorley As main contributors of atomworks, what do you think?

[Croydon-Brixton]

Yes, it'd be an honour - I'm happy to help get it in - anything specific that needs to happen for this? Also as fyi: We're currently still doing a bit of a cleanup of interfaces. So for another ~week some interfaces may still change a bit before they are stable.

[padix-key]

You just need to add it to /doc/extensions.rst.

[nscorley]

We'd be thrilled! We will circle back after a few more days of cleanup so we can integrate when our API's stabilize