Update definition of stage and add definition of task (#1688)
Update definition of stage and add definition of task (#1688)

This PR is a result of a discussion which took place under
another PR. [0] Sending this PR in order to update the Spinnaker
documentation to ensure people who read the documentation can
get the correct information about stages and tasks.

[0] https://github.com/cdfoundation/sig-interoperability/pull/14#discussion_r371995711

Co-authored-by: Dave Dorbin <3a983b921291999521a653180c5b82ccd2c36700@google.com>
spinnaker/spinnaker.github.io,spinnaker/spinnaker.github.io,spinnaker/spinnaker.github.io