DOC: Redirect moved pages
Following  up the discussion in https://github.com/pandas-dev/pandas/issues/24451#issuecomment-450862635 and addressing the concerns of merging the PRs of #24499.

The problem is that we've moved many documentation pages (all the API pages, `release.rst`...) we'd like to move many more (almost every page). And we've got two options if we've been discussion so far are:
- Break all the links in search engines, stack overflow, blog posts... linking to the old locations.
- Add more complexity to the build simiar to https://github.com/pandas-dev/pandas/blob/master/doc/source/conf.py#L246 (which IMO is already quite complex, and just works for specific cases)

After thinking more on it, I think we can build a simpler redirect system than what we have, and keep redirects while reducing the complexity of the build.

What I'd do is create a file (e.g. `doc/source/redirects.csv`) containing the urls of the the old and new url (a line could look like `io,user_guide/io` or `generated/pandas.DataFrame.to_parquet,api/reference/DataFrame/pandas.DataFrame.to_parquet`)

Then, in `doc/source/conf.py` we open and iterate the file, and create a simple html in the old location with a link to the new. May be later we can use a nice template and a javascript that automatically redirects, but I'd keep it simple to start with.

If a page is splitted, we can have more than two columns in the csv (e.g. `io,user_guide/io_plain,user_guide/io_binary`), and show the two links in the redirect page.

I'd also move the current API redirect system to this, and we'd simplify `conf.py`. 

@jorisvandenbossche @TomAugspurger does this sound good?