Update transpiler docs (backport #12731)

[mergify]

Summary

Addresses #12706.

Details and comments

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This is an automatic backport of pull request #12731 done by [Mergify](https://mergify.com).

[qiskit-bot]

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[coveralls]

Pull Request Test Coverage Report for Build 9838044263

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 21 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.01%) to 89.942%

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Files with Coverage Reduction	New Missed Lines	%
crates/qasm2/src/expr.rs	1	94.02%
crates/qasm2/src/lex.rs	8	92.11%
crates/qasm2/src/parse.rs	12	97.15%

Totals
Change from base Build 9807439773:	-0.01%
Covered Lines:	61843
Relevant Lines:	68759

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💛 - Coveralls