Improve header hierarchy for API module pages (backport #12325)

[mergify]

Closes Qiskit/documentation#942. This uses a few principles:

• There should never be more than one h1, as it violates web best practices.
• Functions, classes, and exceptions should always be preceded by at least an h2
  • Why? The docs app generates a header for functions/classes/exceptions, which is important for a) anchor links, b) search, and c) the right page table of contents. But the header will always be an h3 or lower (like h4). It's a semantic violation to go from h1 to h3 - you are supposed to have an intermediate header
• Flatten headers when possible
  • The docs app only shows h2 and h3 in the right page ToC, and it will always be that way because showing h4s makes things too noisy when we experimented. There are a few instances where content is meaningful enough it should show up in the page ToC
  • There were a few instances of multiple headers with only one entry each. This is noisy.
• Use standard RST convention for headers
  • h1 uses = above and below
  • h2 uses = below
  • h3 uses - below
  • h4 uses ~ below

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This is an automatic backport of pull request #12325 done by [Mergify](https://mergify.com).

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[coveralls]

Pull Request Test Coverage Report for Build 8970248533

Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• 23 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.03%) to 89.605%

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Files with Coverage Reduction	New Missed Lines	%
crates/qasm2/src/lex.rs	5	92.11%
crates/qasm2/src/parse.rs	18	95.77%

Totals
Change from base Build 8941650954:	-0.03%
Covered Lines:	62230
Relevant Lines:	69449

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💛 - Coveralls