DOC: fix redirects to user guide in README
Documentation pages were moved so this commit updates those links to
open more directly rather than getting a "This page has been moved
to..." message.

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

thanks @erictleung 