Deprecate Foldable for PlutoUI.details, TwoColumn and ThreeColumn for Columns

[greimel]

In #51 I add a function Columns that can display any number of columns. Its is a drop-in replacement for TwoColumn and ThreeColumn.

Should we remove TwoColumn and ThreeColumn?

In #60 I replace the implementation of Foldable with PlutoUI.details.

Should we remove Foldable and refer to PlutoUI.details in the docs?

[eford]

In #51 I add a function Columns that can display any number of columns. Its is a drop-in replacement for TwoColumn and ThreeColumn.

Should we remove TwoColumn and ThreeColumn?

I suggest that we leave these in the package, label them as deprecated, and replace the demonstrations of them in the example notebook.

In #60 I replace the implementation of Foldable with PlutoUI.details.

Should we remove Foldable and refer to PlutoUI.details in the docs?

I think the function name details is ambiguous. So I'd suggest that we keep the more clear name. Of course, we can credit PlutoUI and say that it's now just a more clear name (and demo) for details.
The one weird thing is that its now a function instead of a type, so the capitalization is confusing. I suppose we could create foldable and declare Foldable deprecated. I'm not really excited about that, but I can see the argument either way.