docs for dtypes

[d-v-b]

we should list all the supported data types prominently in our docs. right now it's basically impossible to find this information.

[dstansby]

Did your 3.1.0 data type refactor help with this at all? I remember you added lots of nice new docs as part of that.

[d-v-b]

the list is here but I think it should probably be more prominent -- it doesn't show up by default in the navbar, because the navbar is too shallow I think