Improve qiskit.circuit.QuantumCircuit page

[abbycross]

Summary

Add code examples where useful, and copyedit if necessary; closes #13299

TODO: determine what needs expanding and/or examples in the attributes/methods on this page

[qiskit-bot]

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

• @Qiskit/terra-core

[coveralls]

Pull Request Test Coverage Report for Build 11506147888

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• 1035 unchanged lines in 45 files lost coverage.
• Overall coverage increased (+0.02%) to 88.672%

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Files with Coverage Reduction	New Missed Lines	%
qiskit/transpiler/passes/scheduling/alignments/pulse_gate_validation.py	1	96.3%
qiskit/transpiler/passes/calibration/pulse_gate.py	1	95.45%
qiskit/pulse/instructions/reference.py	1	93.55%
qiskit/pulse/instructions/directives.py	1	97.14%
crates/accelerate/src/unitary_synthesis.rs	1	92.24%
qiskit/pulse/instructions/play.py	1	97.5%
qiskit/providers/fake_provider/fake_pulse_backend.py	1	95.45%
qiskit/qpy/binary_io/circuits.py	1	91.74%
qiskit/pulse/library/pulse.py	2	83.33%
qiskit/pulse/calibration_entries.py	2	91.19%

Totals
Change from base Build 11461474832:	0.02%
Covered Lines:	74899
Relevant Lines:	84467

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💛 - Coveralls