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 87.95%. Comparing base (38ff2e1) to head (ca02f4e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3768      +/-   ##
==========================================
- Coverage   89.24%   87.95%   -1.29%     
==========================================
  Files         907      907              
  Lines      104866   104866              
==========================================
- Hits        93585    92236    -1349     
- Misses      11281    12630    +1349     

Flag	Coverage Δ
builders	29.21% <ø> (+<0.01%)	⬆️
default	85.86% <ø> (?)
expensive	35.75% <ø> (?)
linux	33.07% <ø> (-55.94%)	⬇️
linux_other	33.07% <ø> (-53.63%)	⬇️
osx	?
win	?
win_other	?

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[blnicho]

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