Update contribution guidelines

[VillePihlava]

Summary

This PR updates the contributing guidelines as discussed in the September 2025 OTP Summit in Helsinki.

Issue

N/A

Unit tests

N/A

Documentation

• CONTRIBUTING.md
• README.md
• .github/pull_request_template.md

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.10%. Comparing base (0e945f9) to head (b18674c).
⚠️ Report is 20 commits behind head on dev-2.x.

Additional details and impacted files

@@              Coverage Diff              @@
##             dev-2.x    #6869      +/-   ##
=============================================
- Coverage      72.10%   72.10%   -0.01%     
+ Complexity     19671    19669       -2     
=============================================
  Files           2125     2125              
  Lines          79533    79533              
  Branches        8051     8051              
=============================================
- Hits           57349    57345       -4     
- Misses         19347    19349       +2     
- Partials        2837     2839       +2     

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[VillePihlava]

The decisions from PR #6866 will have to be referenced in CONTRIBUTING.md once it has been merged

[binh-dam-ibigroup]

This is nicely written and adds much more needed clarity to the expectations for how issues and PRs will be dealt with going forward.

[leonardehrenfried]

If you take a look at Binh's comments, I'm happy to review this.

[leonardehrenfried]

If you take a look at Binh's comments, I'm happy to review this.

[t2gran]

This does not count as a review, but I will file a few suggestions.

In general consider to format the document using 100 characters long lines. This is mainly to make it easy to read in the "source" view.