Use doc_auto_cfg for docs generation

[i509VCB]

This is done when generating docs on docs.rs and locally with RUSTDOCFLAGS="--cfg=docsrs" cargo +nightly doc --all-features

There seems to be some bugs with the labels however:

There does seem to be a rustdoc bug when generating the labels as SwashImage fails to get a feature label automatically assigned to the type, but the functions have the label.

Also FontSystem seems to get an std label even though it is available no_std. I imagine that could be fixed by reworking the reexports.