Add mutation testing results, CI/CD, and customization sections

[kovan]

Summary

• Adds three new sections to the mutation testing guide:
  • Interpret mutation testing results — explains killed/survived/timeout categories
  • Adding mutation testing to your CI/CD workflow — brief integration guidance
  • Customization — documents stryker-config.json with ignore-mutations, ignore-methods, and mutate keys

Changes from original PR

Continues the work from #46064 by @cosminvladutu, incorporating all reviewer feedback from @IEvangelist:

• Applied all 14 inline suggestions (sentence-case headings, US spelling, [\!NOTE] syntax, kebab-case config keys, json fence, etc.)
• Removed unexplained thresholds JSON block per reviewer request
• Fixed ignore-mutations values to use mutation type names (string, logical) per Stryker docs instead of method names
• Added required stryker-config wrapper key in JSON
• Fixed JSON syntax errors (missing comma, indentation)

Test plan

• Verify page renders correctly
• Verify [\!NOTE] alert renders as expected
• Verify JSON code block syntax highlights

---

Internal previews

📄 File	🔗 Preview link
docs/core/testing/mutation-testing.md	Mutation testing