build: move list of builder types mostly into code

[bradfitz]

https://github.com/golang/go/wiki/DashboardBuilders is manually maintained and sometimes goes stale.

I think we should move the descriptions of all new-style coordinator-managed builders into dashboard/builders.go (into a new description field) and then add a new URL like:

https://farmer.golang.org/buildertypes

... which lists them.

Then the wiki page can link to /buildertypes primarily and have a much smaller augmented list, which will eventually be zero.

/cc @crawshaw