Some internal links in the documentation are broken after the
re-organisation because they used absolute URLs. Use :ref:, or at least
:doc: instead and let Sphinx take care of keeping the links up to date.
Also add a redirect for docs we know to have moved, since they may be
linked from other places externally.
Change-Id: I107f40e23a757052da7f5179338f88369a4a72cb
Partial-Bug: #1710084