docs: add drop columns usage page

[avriiil]

This PR adds a usage page about dropping columns

[avriiil]

@MrPowers -- is this the recommended way to drop columns with python deltalake atm?

Using the alter table functionality as recommended here would be better, but AFAICT there is no column drop functionality in dt.alter yet.

[ion-elgreco]

@avriiil let's maybe hold off with this until we have a drop column operation, I'll probably look into this after add column operation is merged

[avriiil]

@ion-elgreco - makes sense! Feel free to ping me when the drop column API is ready and I'll be happy to update this docs page