dev DataFrames into the docs environment

This is suggested by the Documenter documentation.
I was not able to locally run make.jl until I did this.
(Actually the situation was worse because I was running the tests with a different version of the package than the local one, which was yielding confusing error messages.)

docs/Project.toml

@@ -3,6 +3,7 @@ CSV = "336ed68f-0bac-5ca0-87d4-7b16caf5d00b"
 CategoricalArrays = "324d7699-5711-5eae-9e2f-1d82baa6b597"
 Chain = "8be319e6-bccf-4806-a6f7-6fae938471bc"
 DataFrameMacros = "75880514-38bc-4a95-a458-c2aea5a3a702"
+DataFrames = "a93c6f00-e57d-5684-b7b6-d8193f3e46c0"
 DataFramesMeta = "1313f7d8-7da2-5740-9ea0-a2ca25f37964"
 Documenter = "e30172f5-a6a5-5a46-863b-614d45cd2de4"
 Missings = "e1d29d7a-bbdc-5cf2-9ac0-f12de2c33e28"