Clarify content of transpiler passes in documentation

[nathanshammah]

Is your feature request related to a problem? Please describe.
Currently the readme and documentation (user guide, API doc) are quite minimal on the explanation of the transpiler passes and defaults present in ucc.

Describe the solution you'd like
The documentation can be improved with a summary of existing transpiler passes. We want to clarify:

• If the transpiler pass comes from another library, as currently for Qiskit ones, why it was chosen
• If the pass was customized with input variables, e.g.,
• If the pass was modified and why, e.g., VF2
• Choices made in ucc defaults

Additional context
Useful comment in the Discussions #239.

[jordandsullivan]

New section in User Guide on Transpile Passes:

• Clarifies what each pass does that is different from default Qiskit

[bachase]

Perhaps a new consideration in light of #256 is to supplement https://github.com/unitaryfund/ucc/blob/main/docs/source/user_guide.rst#key-modules with more details on why the specific set of passes was chosen for the UCCDefault1 set?

[jordandsullivan]

Yes, that makes sense as a new direction for this part of the documentation. As I recall, these passes were chosen partially by doing some empirical tests of which Qiskit passes seemed to provide the most bang for your buck (i.e. where is most of the gate reduction happening). Is that accurate @sonikaj?

[sonikaj]

Jordan, that's correct and also it comes from consideration of what would be a logical sequence of operations to optimize a general quantum circuit about which not much other information is known. If there is some other information is available at compile time, for example, that it is a chemistry circuit, or it's implementing a Grover's type algorithm, a different sequence of passes, custom to the particular application area, can be designed.