Reorganize the API documentation.

[jsiirola]

Split the API documentation to differentiate between core functionality, beta functionality, and third-party documentation.

Summary/Motivation:

We are starting to get user questions that reference the online documentation (a good thing!); however, they were referencing the Kernel documentation and implying that it would work for general Pyomo models (https://stackoverflow.com/questions/48538945/access-all-variables-occurring-in-a-pyomo-constraint). This change highlights that the Kernel is still "beta" -- mostly because that API should not (yet) be expected to work with models that general users would be creating.

Changes proposed in this PR:

• Reorganize the TOC for the Library Reference to split out beta and third-party documentation.

Legal Acknowledgement

By contributing to this software project, I agree to the following terms and conditions for my contribution:

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.

[qtothec]

It'd be nice if read_the_docs also built relevant PRs.

[jsiirola]

@qtothec: agreed. Unfortunately, we haven't been able to figure out how to make that happen automatically. There appear to be issues tracking this feature request for RTD (readthedocs/readthedocs.org#1340 and readthedocs/readthedocs.org#2465)

[blnicho]

If you view the full .rst file on github it gives you a slightly better preview of what it will look like than just looking at the raw text (minus the special linking done by Sphinx)

[qtothec]

"third" parties

[codecov-io]

Codecov Report

Merging #333 into master will increase coverage by 0.05%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #333      +/-   ##
=========================================
+ Coverage   65.65%   65.7%   +0.05%     
=========================================
  Files         464     465       +1     
  Lines       60428   60534     +106     
=========================================
+ Hits        39673   39776     +103     
- Misses      20755   20758       +3

Impacted Files	Coverage Δ
pyomo/core/kernel/component_matrix_constraint.py	100% <0%> (ø)	⬆️
examples/doc/pyomobook/data-abstract-ch/ABCD7.py	80% <0%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 380933e...f42a918. Read the comment docs.

[jsiirola]

Spoke with @carldlaird, @blnicho, and @whart222: We agreed to merge this to clarify the Library docs, but hold off on any additional changes until after the 5.4 release and #272 is merged (which has a more extensive rework of the documentation hierarchy - in particular to distinguish between User API docs and Developer API docs).