Gate documentation overhaul

[ajavadia]

As part of making the gates into a circuit library...

Adding docstrings for the standard gates including matrix definition, some explanation of what they are, and what they look like in a circuit.

With this I remove the jupyter execute blocks in the documentation that were slowing things down (fixes #3906 )

I also added a RZX gate.

I mostly just changed docstrings so hopefully it won't clash with #3947 or what @Cryoris is doing with gates location.

I did have to change the matrix definition of RYY because the phase factor should not be part of the canonical matrix. For consistency all Pauli rotations are exp(-i.(\theta/2).pauli). And the decomposition is phase correct as well, so no extra phase is required. Only problem is our RZ to U1 unrolling loses the phase, which is something we should fix separately.

[mtreinish]

This should be a new feature section or we should save it for the release. The prelude section is special in that we can only have one for a release since it doesn't really combine multiple elements well since it's a free form paragraph section and not a list. Typically before a release I've been writing a small blurb summarizing the big changes for the release. This release is turning out to be quite large (not by patch count but because of several substantial changes) so we'll probably have to collaborate on a good prelude for the release.

[ajavadia]

yeah i put it at the top here so we don't forget to write a prelude about documentation that came out of the docathon. i figured we can tweak it towards the release. but documentation doesn't quite fit a "feature" in my mind

[Cryoris]

This creates merge conflicts in all the standard gate files in qiskit/extensions/standard since I'm working on a PR to move them to qiskit/circuit/gates. But since you only changes the docstrings this is fixable 👍

[Cryoris]

That's a very much needed upgrade of the documentation and probably helps many people that are confused with different bit orderings! Also we don't have to go look for the gate definitions elsewhere anymore if we need a reminder on the matrix repr 👍

Some brackets might not be perfect, since fractions are enclosed without \left \right but we can do a second run of small improvements on the docs later.