Reorganize transpiler modules in API docs (cherry-pick of #1645)

[Eric-Arellano]

Closes Qiskit/documentation#1240 and partially addresses Qiskit/documentation#942 for runtime.

Reduces transpiler modules

Before:

module	status
qiskit.transpiler	only links to passes
qiskit.transpiler.passes	only links to basis and scheduling

Now:

module	status
qiskit.transpiler.passes	links to ConvertIdToDelay and scheduling

This saves the user clicks. Now each page is useful enough to stand on its own.

This is what the index page now looks like:

This change assumes that we will not add new modules to qiskit.transpiler for some time, such as qiskit.transpiler.my_module. If we do eventually add a new module, then we can easily revert transpiler.rst to point to qiskit_ibm_runtime.transpiler again.

This change also assumes that we won't lots of new APIs to qiskit.transpiler.passes.basis. If we do, we would want to consider going back to qiskit.transpiler.passes.basis being its own standalone page. We can do that easily if necessary in the future.

Improves header hierarchy

Options

Before, we were using bold text to act like headers. Instead, we should be using proper headers for styling, SEO, and accessibility (screen readers).

qiskit_ibm_runtime.transpiler.passes.scheduling

This PR moves the classes list to be higher in the file and fixes some of the other headers to higher heading levels.

Removes unused docstrings

The utils module is not exposed, and Kevin said it should not be since it's internal. So I deleted its docstring to make this more clear.

[coveralls]

Pull Request Test Coverage Report for Build 8905113783

Details

• 3 of 3 (100.0%) changed or added relevant lines in 3 files are covered.
• 92 unchanged lines in 6 files lost coverage.
• Overall coverage increased (+0.09%) to 83.704%

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Files with Coverage Reduction	New Missed Lines	%
qiskit_ibm_runtime/estimator.py	2	97.62%
qiskit_ibm_runtime/api/clients/base_websocket_client.py	2	90.43%
qiskit_ibm_runtime/base_runtime_job.py	2	77.67%
qiskit_ibm_runtime/base_primitive.py	9	95.34%
qiskit_ibm_runtime/session.py	10	85.04%
qiskit_ibm_runtime/qiskit_runtime_service.py	67	78.89%

Totals
Change from base Build 8695950048:	0.09%
Covered Lines:	6277
Relevant Lines:	7499

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💛 - Coveralls