feat: add repro guide and needs-repro automation

[jamescrosswell]

Summary

Resolves #4105

• Add info on how to provide minimal repro #4105

Changes

• .github/repro.md – New guide explaining what a minimal reproduction is, why we ask for one, how to create it, and what to avoid. Adapted from the .NET MAUI repro guide but tailored for the Sentry .NET SDK.

• .github/ISSUE_TEMPLATE/bug_report.yml – Added a markdown block at the top of the bug report form linking to the new guide, and updated the "Steps to Reproduce" field description to reference the guide.

• .github/workflows/needs-repro.yml – New GitHub Actions workflow that automatically posts a comment with repro instructions and a link to the guide whenever the s/needs-repro label is added to an issue.

#skip-changelog

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Semver Impact of This PR

⚪ None (no version bump detected)

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📖	Do not forget to update Sentry-docs with your feature once the pull request gets approved.

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.89%. Comparing base (2079493) to head (c2476f6).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4975      +/-   ##
==========================================
+ Coverage   73.86%   73.89%   +0.02%     
==========================================
  Files         496      496              
  Lines       17951    17951              
  Branches     3516     3516              
==========================================
+ Hits        13259    13264       +5     
+ Misses       3833     3825       -8     
- Partials      859      862       +3     

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