Update Pauli Twirling user guide

[purva-thakre]

With the changes made in #2454, the user guide for PT is still incomplete.

Below are the things that still require some work before we remove the WIP warning at the top of each page in this user guide.

• Update the workflow diagram image
• The Advantages and Disadvantages section of the When should I use PT? are missing details on coherent vs incoherent noise.
• On What additional options are available when using PT? make a table showing which noise channels will be tailored by PT (preferably showing the PTM heatmaps for each).
• The What additional options are available when using PT? is missing a subsection on how the number of generated Pauli twirled circuits affects the outcome.
• In What additional options are available when using PT? add a subsection on how PT could be stacked with some QEM technique.
• A new workflow diagram for stacking PT and a QEM technique
• Portions of What happens when I use PT? are commented out.
• What is the theory behind Pauli Twirling? could use additional details and clarifications.
• Also, use this issue to utilize add_noise_to_two_qubit_gates somewhere in the docs. We don't need to expand the dictionary but an example showing how a person could add their desired noise channel by updating the dictionary should be more than enough.