Increase documentation of built-in transpiler plugins

[jakelishman]

Summary

This overhauls how the qiskit.transpiler documentation talks about the transpiler plugins. All of the built-in plugins now have a decent amount of overview documentation, and the requirements and expectations for each stage of the preset pipelines is explained in more detail.

This form of the documentation makes the distinction between "a compilation routine in general" and "Qiskit's specific choice of default pipeline" clearer, to avoid confusion for advanced users.

Much of the guide-level explanations of the different preset pipeline stages moved to https://docs.quantum.ibm.com some time ago, so this PR removes those, in favour of focussing on the actual API, and inserts links to learn more about the principles elsewhere.

The guide-level explanation of scheduling is left in-place for now, because the content on the other parts of the IBM documentation isn't as complete for that.

Details and comments

I've attempted to draw a good line between explaining what Qiskit actually does, so users know, and maintaining that we reserve the right to modify specifics of things (especially default plugins) between minor versions.

This isn't a full rewrite of everything - I feel like we could probably do with revisiting the documentation of a lot of individual transpiler passes' classes - but I was writing my own plugin over the last month or so, and this is documentation I personally wanted to see.

[qiskit-bot]

One or more of the following people are relevant to this code:

• @Qiskit/terra-core

[jakelishman]

Docs team: as of right now, we don't necessarily need to bother copy-editing - first I need to get sign-off from the rest of the compiler team that this is the appropriate level of detail for us to document, and doesn't over-tie our hands with respect to the stability policy. If you guys have any comments about the high-level content, that's very welcome immediately.

[coveralls]

Pull Request Test Coverage Report for Build 12657127421

Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• 6 unchanged lines in 3 files lost coverage.
• Overall coverage increased (+0.002%) to 88.95%

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Files with Coverage Reduction	New Missed Lines	%
crates/accelerate/src/unitary_synthesis.rs	1	93.18%
crates/qasm2/src/expr.rs	1	94.02%
crates/qasm2/src/lex.rs	4	92.48%

Totals
Change from base Build 12655273978:	0.002%
Covered Lines:	79456
Relevant Lines:	89327

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💛 - Coveralls