DOC: add redirects to ensure old links instead of building the docstring page
We still have a lot of API pages with 'old' namespace usage (eg `pandas.io.parsers.read_csv` instead of `pandas.read_csv` as how it should show up in the documentation). Now we build the pages as well, this PR changes this in adding redirect pages.

(The reason I care about this is that googling eg `read_csv` still often turns up the old page (although I don't know how fast this disappears when we would just remove it) and these are therefore used a lot on eg SO)
