Generate doc pages summarizing AD backend compatibility

[adrhill]

This could be used by package developers to show users which backends their functions are compatible with.
Could be called in runtests.jl, docs/make.jl or in CI.

[gdalle]

I would close this in favor of #109 because generating markdown formatted stuff will only be useful to those users who want it exactly the same way we do. A table format is much more flexible