Autogenerate API documentation

[jsiirola]

Fixes # .

Summary/Motivation:

This PR is primarily adds the automatic generation of the API documentation for the entire Pyomo codebase to the Sphinx build.

This also silences all Sphinx warnings that were being generated

Changes proposed in this PR:

• Add the API documentation under Reference / Library Reference
• Move the old library reference to Reference / Topical Reference
• Switch to using BiBTeX alpha format for all references (that are not references to Pyomo publications)
• Consolidated references into the bibliography
• Remove the (unused) Sphinx intersphinx reference
• Remove the use of sphinx.ext.autosectionlabel
• Restored the working with abstract models" docs
• Extensive updates to docstrings to resolve Sphinx warnings
• Pyomo code
  • cleaned ConfigDict numpydoc generation (improve references, resolve sphinx warnings)
  • Add an IntEnum wrapper to rewrite some docstrings to silence Sphinx warnings
  • remove use of "bare" absolute imports in Pyomo __init__.py files (resolves Sphinx autosummary issue)
  • Prevent importing Qt when building docs
  • Switch from docstrings to decorators for defining PLY grammars

Legal Acknowledgement

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1. I agree my contributions are submitted under the BSD license.
2. I represent I am authorized to make the contributions and grant the license. If my employer has rights to intellectual property that includes these contributions, I represent that I have received permission to make contributions and grant the required license on behalf of that employer.

[mrmundt]

Not relevant for this PR but marking for posterity - what is n_scen?

[jsiirola]

I believe "number of scenarios"

[mrmundt]

Yes yes, I am pleased.