Improve qiskit.circuit.QuantumCircuit page

[abbycross]

Summary

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

• QuantumCircuit
• data
• global_phase
• name
• metadata
• qregs
• cregs
• qubits
• ancillas
• clbits
• layout
• op_start_times
• num_ancillas
• num_clbits

Moving the rest to a new PR to break up the work in more manageable pieces!

[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 12892391120

Details

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• 12 unchanged lines in 4 files lost coverage.
• Overall coverage decreased (-0.002%) to 88.925%

---

Files with Coverage Reduction	New Missed Lines	%
crates/accelerate/src/two_qubit_decompose.rs	1	92.06%
crates/qasm2/src/expr.rs	1	94.23%
crates/qasm2/src/lex.rs	4	92.23%
crates/qasm2/src/parse.rs	6	97.61%

Totals
Change from base Build 12891072018:	-0.002%
Covered Lines:	79462
Relevant Lines:	89358

---

💛 - Coveralls

[frankharkins]

Thank you! I have some minor suggestions but generally it looks great. I think this'll help a lot; seeing a concrete example of what the output looks like is often a big help for me.

[qiskit-bot]

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

• @Qiskit/terra-core