Add LLM/AI code disclosure section to PR template

[IAlibay]

Added a section for LLM/AI generated code disclosure in the PR template.

Please note that I'm not adding a tickbox but rather a yes/no, to avoid the case where someone doesn't tick it and we don't know if they forgot or if they just aren't using an LLM.

PR Checklist

• Issue raised/referenced?
• Tests updated/added?
• Documentation updated/added?
• package/CHANGELOG file updated?
• Is your name in package/AUTHORS? (If it is not, add it!)

Developers Certificate of Origin

I certify that I can submit this code contribution as described in the Developer Certificate of Origin, under the MDAnalysis LICENSE.

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📚 Documentation preview 📚: https://mdanalysis--5196.org.readthedocs.build/en/5196/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.72%. Comparing base (1e7644f) to head (2a76be3).
⚠️ Report is 31 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #5196      +/-   ##
===========================================
- Coverage    92.72%   92.72%   -0.01%     
===========================================
  Files          180      180              
  Lines        22472    22472              
  Branches      3188     3188              
===========================================
- Hits         20838    20837       -1     
- Misses        1176     1177       +1     
  Partials       458      458              

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