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 #1469

[qiskit-bot]

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• @frankharkins

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[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!

[shraddha-aangiras]

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

[abbycross]

@shraddha-aangiras do you think you can finish this PR? Alternatively, we can close and re-open in a fresh PR.

[kaelynj]

Moved to #3989