DOC: add redirects for moved api pages

[jorisvandenbossche]

Due to the changes in the directory layout, some API changes have moved. We should add redirects for those.

I think is this is mainly about Resampler (off the top of my head)

[jorisvandenbossche]

and the styler ones

[TomAugspurger]

@jorisvandenbossche do you have any info on how to do redirects with sphinx? I'm not finding much. Or would we do this at a different level?

[jorisvandenbossche]

We already have some machinery: https://github.com/pandas-dev/pandas/blob/master/doc/source/conf.py#L231 but you will have to adapt it a little bit (currently they are for top-level ones). Probably fine to remove the existing ones as well.