Document how to use high level synthesis plugins

[mtreinish]

Summary

This commit adds missing documentation to the transpiler synthesis plugin page on how to use the high level synthesis plugin interface. There was existing documentation on how to write a high level synthesis plugin but not for how users can consume any installed plugins. This commit fills that documentation gap and also adds the HLSConfig class to the documentation as a stable defined interface. This was an oversight as we've not been treating the API interface as stable as it's the primary entrypoint for using high level synthesis plugins. Additionally, a new function high_level_synthesis_plugin_names is added in this PR to make querying the installed plugins a bit easier as to do this before you had to instantiate a plugin manager object to do this. This makes the querying interface similar to what exists for unitary synthesis.

Details and comments

Fixes #11356
Fixes Qiskit/documentation#505

[qiskit-bot]

One or more of the the following people are requested to review this:

• @Qiskit/terra-core

[coveralls]

Pull Request Test Coverage Report for Build 7213117378

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• 3 of 5 (60.0%) changed or added relevant lines in 2 files are covered.
• 6 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.01%) to 87.564%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
qiskit/transpiler/passes/synthesis/plugin.py	2	4	50.0%

Files with Coverage Reduction	New Missed Lines	%
qiskit/pulse/parameter_manager.py	1	96.84%
crates/qasm2/src/lex.rs	5	91.67%

Totals
Change from base Build 7163309461:	0.01%
Covered Lines:	59784
Relevant Lines:	68275

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💛 - Coveralls

[jakelishman]

I'll take over the last few changes needed for this on Matt's behalf, since he's on holiday now and I'd like to get the documentation whole plugged sooner rather than later. Let me know if there's anything more that needs changing, and I can do it.

[kevinsung]

Thanks, it's pretty clear now. Now the documentation here is clearer than in the docstring of HLSConfig. How about we add a link in that docstring to this documentation?

[kevinsung]

LGTM with just one comment about improving the docstring of HLSConfig itself

[jakelishman]

Sure, I've tried to make HLSConfig a little clearer about the allowed forms, and put in a cross-link to the overview section in 5cc74e1.

[kevinsung]

Thanks!

[jakelishman]

(proxying approval)