Cleaned up documentation.

Removed some Inktank references, mentioned the split of tasks (between
teuthology/task and chef-qa-suite/tasks), fixed some wording, removed
chef reference that no longer applies.

Did not update the teuthology-suite section at the end of this file
because further documentation changes are coming.

Fixes: #9292
Signed-off-by: Warren Usui <warren.usui@inktank.com>