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: #2075 (closed)