doc: add notification_reason to docs

[Spaceman1701]

Which user-facing changes does this PR introduce?

[BUGFIX] doc: fix missing `notification_reason` field in webhook documentation

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Enterprise

Run ID: 40ace8a3-f2a1-4045-9fd4-2074d7bd0363

📥 Commits

Reviewing files that changed from the base of the PR and between d4284cf and 7749dd9.

📒 Files selected for processing (1)

• docs/configuration.md

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📝 Walkthrough

Walkthrough

Updated the Alertmanager webhook receiver JSON example to include a top-level notification_reason field.

Changes

Alertmanager webhook docs

Layer / File(s)	Summary
Webhook JSON example docs/configuration.md	Adds notification_reason to the webhook POST JSON example as a top-level field.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~2 minutes

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Description check	⚠️ Warning	The description includes release notes but omits the checklist items, issue links, and sign-off/status details required by the template.	Add the full PR checklist, list any related issue links, and confirm sign-off and applicable tests or documentation updates.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title is concise, follows the requested area: description format, and matches the documentation update.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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