make some columns into factors for parsnip find table

[EmilHvitfeldt]

The “package” field in the “all of tidymodels” table has a drop-down/scrollable entry list, which is nice, and I think due to that column being a factor rather than character. It’d be nice to have this for model types, engines and modes as well—could we convert those to factor? The remaining string columns make sense as strings.

_Originally posted by @simonpcouch in https://github.com/tidymodels/tidymodels_dot_org/pull/4#pullrequestreview-1253586411_

[EmilHvitfeldt]

If you try the naive way, you get this weird artifact, because the data is pre-formatted. I will look into this later when I get to styling!