Qiskit Pulse documentation revamping

[TsafrirA]

The documentation of Qiskit's Pulse module is in dire need of revision. This issue will try to track progress and problems.

To name some of the obvious problems in the API docs:

• The overview section is overcrowded and unorganized, with some sub-modules given in excruciating detail.
• The list on the navigation panel has no meaningful structure, and includes a mix of objects of significantly different importance.

It should be noted that the tutorial covering Pulse also reflects some of the problems mentioned above.

There is a caveat - with the majority of Qiskit's Pulse development headed over to the feature branch, and substantial API changes coming, one has to be careful not to conflict with the ongoing development process. Sections calling for a rebuild based on a more holistic view (overview, tutorial) should probably wait until the API picture clears a bit more.