Add AI policy to contribution guide

[blnicho]

Summary/Motivation:

This PR adds an AI contribution policy to our contributor guide based on discussions we've had at the Pyomo developer meetings. This is meant to be a "living" policy that will be regularly updated as the tools and our observations evolve. Feedback and comments are welcome.

Changes proposed in this PR:

• Add AI contribution policy to the contributor guide
• Update PR template with an "AI-Use Disclosure" section (demonstrated below)

AI-Use Disclosure

• AI tools were NOT used during the preparation of this PR

or

• AI tools contributed to the development of this PR

  • AI tools generated documentation (including the PR description/comments, code comments, and/or Sphinx documentation)
  • AI tools generated tests (baselines, examples, and/or code)
  • AI tools generated code (apart from tests)

  Review process (select ONE):

  • Rewritten: All AI-generated content was rewritten by me before being committed.
  • Reviewed/verified: I retained AI-generated content and verified it before committing. Verification included (as applicable):
    • Ran the code and fixed issues
    • Added and ran tests
    • Checked correctness/logic of code and tests
    • Checked for alignment with the contribution guide
    • Considered security implications
  • As-is: AI-generated content was commited directly to the repository

Notes for reviewers (optional):

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 90.17%. Comparing base (2a8e709) to head (20c146e).
⚠️ Report is 105 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3959      +/-   ##
==========================================
+ Coverage   90.11%   90.17%   +0.06%     
==========================================
  Files         908      909       +1     
  Lines      108378   109990    +1612     
==========================================
+ Hits        97660    99182    +1522     
- Misses      10718    10808      +90     

Flag	Coverage Δ
builders	29.11% <ø> (-0.01%)	⬇️
default	86.09% <ø> (?)
expensive	35.14% <ø> (?)
linux	87.62% <ø> (-1.99%)	⬇️
linux_other	87.62% <ø> (+<0.01%)	⬆️
oldsolvers	28.07% <ø> (+<0.01%)	⬆️
osx	83.07% <ø> (+0.01%)	⬆️
win	85.37% <ø> (+<0.01%)	⬆️

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