Introduce formal principle documentation to support contrib changes

[mrmundt]

Fixes None but starts to address #2460

Summary/Motivation:

Stage 1 of the changes inside pyomo.contrib is documenting a lot of different development principles that make the change a lot more reasonably understandable. This PR creates the new directories with their basic READMEs as well as creates documentation about our overarching development principles, including a teaser to the upcoming changes in contrib as part of their contents.

Changes proposed in this PR:

• Create basic structure for addons and devel
• Create new principles.rst documentation
• Update the testing guidance because it was missing something kind of essential
• (Tiny extra fix) Include doc/OnlineDocs/api in .gitignore because I have almost accidentally committed those files WAY too many times

Legal Acknowledgement

By contributing to this software project, I have read the contribution guide and 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.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.46%. Comparing base (f2d23a5) to head (dc97bbc).
⚠️ Report is 29 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3768      +/-   ##
==========================================
- Coverage   89.48%   89.46%   -0.02%     
==========================================
  Files         904      899       -5     
  Lines      105385   105370      -15     
==========================================
- Hits        94299    94266      -33     
- Misses      11086    11104      +18     

Flag	Coverage Δ
builders	29.04% <ø> (-0.01%)	⬇️
default	83.57% <ø> (?)
expensive	35.52% <ø> (?)
linux	86.76% <ø> (-2.45%)	⬇️
linux_other	86.76% <ø> (+<0.01%)	⬆️
oldsolvers	29.68% <ø> (-0.01%)	⬇️
osx	82.91% <ø> (-0.01%)	⬇️
win	84.98% <ø> (-0.02%)	⬇️
win_other	84.98% <ø> (-0.02%)	⬇️
xpress95	?

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[blnicho]

I have a bunch of minor edit suggestions but overall I think this looks great!

[michaelbynum]

I'm only about halfway done, but here are a couple comments/questions.

[michaelbynum]

I have some minor comments, but I think this looks great overall.

[emma58]

This looks nice--I put a lot of ideas in the comments.

[jsiirola]

Suggested change

	The `pyomo.addons` directory contains **mostly stable extensions** to Pyomo
	The `pyomo.addons` directory contains **stable extensions** to Pyomo

[mrmundt]

I disagree. We explicitly say "mostly stable" everywhere else we talk about addons. This was intentional language to follow the idea of "some level of promised stability," but they still might change in small ways.

[blnicho]

I found an issue with how some of the code snippets are rendered and I have a few minor edit suggestions.