doc: Updated the upgrade guide for Aronaut and Bobtail to Cuttlefish.

fixes: #4874

Signed-off-by: John Wilkins <john.wilkins@inktank.com>
doc: Fixed a few typos.

Signed-off-by: John Wilkins <john.wilkins@inktank.com>
doc: Reordered header levels for visual clarity.

Signed-off-by: John Wilkins <john.wilkins@inktank.com>
doc: Incorporating Tamil's feedback.

Signed-off-by: John Wilkins <john.wilkins@inktank.com>
Merge remote-tracking branch 'gh/wip-doc-cuttlefish' into next