am a6602f1f: Add docs for SDK update. Add redirects for preview files. Manually integrate a few doc changes from cupcake.

Merge commit 'a6602f1fe4590e48c760f21ce29a92629240c463' into eclair

* commit 'a6602f1fe4590e48c760f21ce29a92629240c463':
  Add docs for SDK update.