Add definitions for prob_meas0_prep1, prob_meas1_prep0, and readout_length

[SiddharthRC]

This should add in definitions for prob_meas0_prep1, prob_meas1_prep0, and readout_length, along with corresponding clusters of code to the documentation.

[qiskit-bot]

Thanks for contributing to Qiskit documentation!

Before your PR can be merged, it will first need to pass continuous integration tests and be reviewed. Sometimes the review process can be slow, so please be patient. Thanks! 🙌

One or more of the following people are relevant to this code:

• @abbycross
• @frankharkins
• @lerongil
• @mtreinish

[review-notebook-app]

Check out this pull request on 

See visual diffs & provide feedback on Jupyter Notebooks.

---

Powered by ReviewNB

[CLAassistant]

All committers have signed the CLA.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[Eric-Arellano]

Hey Siddharth, pardon the delay. We had a big deadline this month so fell behind on review. I've asked a coworker to take a look when she gets back from vacation.