docs/features: add zink features

docs/features: add zink features

This is base on the exported extension strings, with some known-bad
extensions removed. There might be more that should be removed, as their
support isn't per-spec, but this gives us some more information, at
least.

There's also a few features that seems to be trivial to enable, simply
by flipping a cap. But let's document what is expected to work first.

Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/2075
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4963>