List plugin-supported deploy targets in /docs/deployment/

[nixjdm]

As pointed out here it is not obvious enough that there are other deployment targets, and you can't see them at a glance in this prime spot. Especially noteworthy is the popular S3 plugin.

I would prefer a programmatic way of grabbing these plugins to list them in this location if it isn't too convoluted, but that might be more of a hassle than it's worth.