use page redirects from old structure to new structure docs

[PieterGit]

I noticed some of my bookmarks don't work because of the doc reorg.
To prevent that (and to prevent broken links from search engines) we could use page redirects, see http://docs.readthedocs.io/en/latest/user-defined-redirects.html

[danamlewis]

Good call. Just did a bunch of redirects in RTD based on the old left hand nav, so should have gotten 90%+ of the old main links people were using. Closing this, but do PM me any links that need redirecting & I can do those as well.