Improve documentation on Registers and Qubits

[shraddha-aangiras]

Based on user feedback from the Qiskit Slack, I added more explanations of commonly used methods and attributes of circuits. In particular:

• explained how to extract information from circuits
• added information on creating and managing registers
• added information measuring circuits
  fixes Better documentation on Registers and Qubits #1469

[qiskit-bot]

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

• @frankharkins

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[javabster]

Thanks so much, this is a lovely contribution! I just left a couple of minor comments but other than that its looking really good

[javabster]

I believe in the original issue attached to this PR it was mentioned that a user was especially interested in the qc.qubits.index(your_qubit) method of getting information. could you include something about this as well as the find_bit method?

[shraddha-aangiras]

I didn't include that due to this comment in a previous PR which says it's not recommended due to the significant performance overhead. Please let me know whether to include it or not in reference to this, thanks!

[frankharkins]

I agree with @shraddha-aangiras. The two approaches have the same output, but find_bit is more efficient, so I don't think we should suggest using index.

[frankharkins]

@shraddha-aangiras we've recently edited the history on main, can you please reset your branch using the following commands.

# Make sure you're on your branch
git switch improvedocs-issue1469
# Overwrite your branch with our history
git reset --hard origin/main
# re-apply your commits on top of the new history
git cherry-pick c370ad180de77c0b816dc025dd719806112f753a  361ac3ffb84190f25930a9c6a94819f4eb058c9f
# Check everything looks correct
git diff main
# If all good, force-push to your branch
git push --force

Let me know if you have any questions. Sorry for the inconvenience.

[shraddha-aangiras]

@frankharkins No problem, will get that along with the suggested changes (thanks @javabster!) fixed by tomorrow

[shraddha-aangiras]

@frankharkins can you let me know if I did it right? Thanks!

[frankharkins]

I agree with @shraddha-aangiras. The two approaches have the same output, but find_bit is more efficient, so I don't think we should suggest using index.

[frankharkins]

@shraddha-aangiras looks great! I have a few suggestions.

Also, sorry about my instructions, but glad you worked it out. (For others reading: git reset --hard origin/main only works if "origin" refers to this repo, which is probably not the case for external contributors.)

[kaelynj]

This is a great contribution thank you! This satisfies what I was thinking of when I opened #1601

The only thing I'd ask before approving is if you could run the notebook linter to make sure all of the formatting is consistent. You'll just need to run
tox -e fix -- docs/**/*.ipynb if you have tox installed.

[beckykd]

This is great! Thank you so much for your contribution!

[beckykd]

In what order? The order in which they were defined?

[shraddha-aangiras]

Clarified this in the latest commit, thank you!

[shraddha-aangiras]

Thanks @beckykd @kaelynj, made the changes you suggested :)

[beckykd]

Suggested change

	"## Measure qubits\n",
	"<span id=\"measure-qubits\"></span>\n",
	"## Measure qubits\n",

[kaelynj]

I would suggest changing this first couple sentences to:

Measurements are used to sample the states of individual qubits and transfer the results to a classical register. Note that if you are submitting circuits to a Sampler primitive, measurements are required. However, circuits submitted to an Estimator primitive must not contain measurements.

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