Reorganize CI jobs and stages (#5983)
Reorganize CI jobs and stages (#5983)

This commit reorganizes the stages and job structure of our CI jobs to
try and improve overall project throughput. It moves to a 3 stage
pipeline instead of the 2 we used before. The first stage runs python
3.7 on all OSes and lint, the second runs the remaining unittest jobs
(for 3.6, 3.7, and 3.8), and the final stage runs the docs builds and
tutorials job (which are the slowest jobs). The goal is by splitting
thing up into more stages and making the slowest jobs run last that this
will improve overall efficiency of CI.

Co-authored-by: Kevin Krsulich <78cdb98aeddd45630046ff39f0a19d75c82aaaa7@ibm.com>
Co-authored-by: mergify[bot] <b09a6ee808b67e98a221404e7aaa52e0398a4954@users.noreply.github.com>
QISKit/qiskit-sdk-py,QISKit/qiskit-sdk-py,QISKit/qiskit-sdk-py