Add missing docstrings for `EquivalenceLibrary`

[raynelfss]

Summary

The following commits attempt to fix #13258 by adding all of the missing docstrings that were lost after #12585.

Details and comments

Add missing docstrings in the following methods:

• Python (PyO3):
  • EquivalenceLibrary struct class.
  • EquivalenceLibrary::get_graph method.
  • EquivalenceLibrary::py_keys method.
  • EquivalenceLibrary::py_node_index method.
• Rust:
  • EquivalenceLibrary::keys method.

[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 11709092718

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

• 0 of 2 (0.0%) changed or added relevant lines in 1 file are covered.
• 2385 unchanged lines in 129 files lost coverage.
• Overall coverage decreased (-0.1%) to 88.774%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
crates/accelerate/src/equivalence.rs	0	2	0.0%
<!--	Total:	0	2

Files with Coverage Reduction	New Missed Lines	%
qiskit/transpiler/passes/scheduling/alignments/pulse_gate_validation.py	1	96.3%
qiskit/circuit/library/iqp.py	1	96.15%
qiskit/transpiler/passes/calibration/pulse_gate.py	1	95.45%
qiskit/circuit/library/boolean_logic/quantum_or.py	1	98.08%
qiskit/pulse/instructions/reference.py	1	93.55%
qiskit/circuit/library/standard_gates/u.py	1	93.0%
qiskit/circuit/library/boolean_logic/quantum_and.py	1	97.96%
qiskit/synthesis/unitary/qsd.py	1	97.58%
qiskit/pulse/instructions/directives.py	1	97.14%
qiskit/circuit/library/boolean_logic/inner_product.py	1	96.0%
<!--	Total:	2385

Totals
Change from base Build 11219438971:	-0.1%
Covered Lines:	77108
Relevant Lines:	86859

---

💛 - Coveralls