Rework ConfigDict numpydoc generation support

[jsiirola]

Fixes # .

Summary/Motivation:

This PR is a significant overhaul of the utilities for converting ConfigDict objects unto numpydoc documentation strings. This was motivated by #3641, where it was pointed out that it is difficult / impossible to cleanly document CONFIG class attributes and nested ConfigDict objects.

This PR demonstrates the new use / formatting using the NLWriter and the contrib.solver Ipopt interface. Promulagating it to the rest of the solvers / transformations is left for a future PR.

Changes proposed in this PR:

• Move the detailed (explanation) documentation of the Config system into the RST file (prevents duplicate labels / references
• Add new decorators:
  • @document_configdict for documenting classes derived from ConfigDict
  • @document_class_CONFIG for documenting CONFIG class attributes (and optionally, methods that use CONFIG for keyword argument resolution
• monkeypatch Sphinx so that non-class domain validators are correctly linked in the documentation.
• Update the numpydoc formatter:
  • reference (do not recurse into) nested ConfigDict if the ConfigDict is documented separately (either through @document_configdict or @document_class_CONFIG)
  • Avoid unused (and unneeded) dict object in ConfigDict domain
• Rework the Executable and Library factories to be true singletons; support their use as domains in ConfigValue
• Convert repn.plugins.nl_writer and contrib.solver.solvers.ipopt to the new system (as demonstrators / test cases).

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[mrmundt]

I overall love this - just found some wee typos / one nitpick.

[mrmundt]

There is a bit of back-and-forth across these docks - do you want it to be capitalized or not? (Config or config)

[blnicho]

This is really neat! I have a couple minor questions but otherwise I think this looks good.