Expand discriminator documentation

[coruscating]

Currently the docs don't explain how to use MultiStateDiscrimination on states beyond the |0>, |1> space by passing in the right schedule. This seems like a good fit for an experiment manual, perhaps by showing an example that calibrates the x12 pulse using the EFRoughXSXAmplitudeCal experiment. The current link to the Qiskit textbook also needs to be updated to the new URL: https://learn.qiskit.org/course/quantum-hardware-pulses/accessing-higher-energy-states-with-qiskit-pulse