docs: Fixing redirects for DAC-on-devsite

On Devsite, the redirect parser doesn't understand regular
expressions, which breaks ~ 10% of our existing redirects; fixing
them now. Also fixing a few other redirect-related bugs while I've got
the hood open.

See first comment for doc stage location.

bug: 28447522
bug: 28467616

Change-Id: I5f315fd54cb9353a56674b5aac4dcd53875263c9