Improve documentation for InstructionDurations class

[phalakbh]

Summary

This PR improves the documentation for the InstructionDurations class with comprehensive documentation and clearer examples. The changes include a detailed class docstring explaining the three-tier lookup system (parameterized → qubit-specific → default), comprehensive __init__ documentation with practical examples showing all tuple formats, and explanations of when qubits and parameters can be None to create fallback behaviors.

Details and comments

Fixes #14691
I am still not too sure on how to get the InstructionDurationType alias to reflect the type hint as discussed in the issue, so for a start I have specified in the doc string within __init__ what each tuple in the list should look like, along with the data types.

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[coveralls]

Pull Request Test Coverage Report for Build 16505924737

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.

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• 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.
• 76 unchanged lines in 5 files lost coverage.
• Overall coverage decreased (-0.03%) to 87.747%

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Files with Coverage Reduction	New Missed Lines	%
crates/qasm2/src/lex.rs	2	92.53%
crates/transpiler/src/passes/elide_permutations.rs	3	94.92%
crates/transpiler/src/passes/gate_direction.rs	11	96.05%
crates/qasm2/src/parse.rs	12	97.09%
crates/transpiler/src/target/mod.rs	48	84.99%

Totals
Change from base Build 16406114169:	-0.03%
Covered Lines:	81426
Relevant Lines:	92796

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💛 - Coveralls