docs: describe Alertmanager architecture

[locker95]

Summary

Keep a short, stable summary and documentation link below the architecture diagram. Add an architecture overview to docs/alertmanager.md covering the alert flow, while linking to the existing high-availability documentation for cluster-specific behavior.

This keeps implementation details out of the README so they are less likely to become stale.

cc @simonpasquier

Pull Request Checklist

• Please list all open issue(s) discussed with maintainers related to this change
  • Fixes Add description for Architecture diagram in README #2424
• Is this a new Receiver integration?
  • I have already tried to use the Webhook Receiver Integration and 3rd party integrations before adding this new Receiver Integration
• Is this a bugfix?
  • I have added tests that can reproduce the bug which pass with this bugfix applied
• Is this a new feature?
  • I have added tests that test the new feature functionality
• Does this change affect performance?
  • I have provided benchmarks comparison that shows performance is improved or is not degraded
  • I have added new benchmarks if required or requested by maintainers
• Is this a breaking change?
  • My changes do not break the existing cluster messages
  • My changes do not break the existing api
• I have added/updated the required documentation
• I have signed-off my commits
• I will follow best practices for contributing to this project

Validation

• git diff --check
• Verified the README architecture link resolves to docs/alertmanager.md#architecture
• Verified the architecture overview links to docs/high_availability.md

Which user-facing changes does this PR introduce?

NONE

[coderabbitai]

📝 Walkthrough

Walkthrough

README.md and docs/alertmanager.md now document Alertmanager’s alert flow, architecture, high-availability replication, and alert-limit metric labeling.

Changes

Alertmanager documentation

Layer / File(s)	Summary
Architecture flow description README.md, docs/alertmanager.md	Adds descriptions of alert ingestion, storage, grouping, routing, silences, inhibition, notification delivery, and high-availability replication.
Alert-limit metric description docs/alertmanager.md	Updates the description of the alert-names-in-metrics feature flag and its alertname label.

Estimated code review effort: 1 (Trivial) | ~2 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	The changes satisfy #2424 by adding a brief architecture description for the README diagram and linking to the detailed docs.
Out of Scope Changes check	✅ Passed	The modified README and Alertmanager docs align with the stated documentation goal and do not introduce unrelated scope.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Title check	✅ Passed	The title is concise and accurately summarizes the main change: documenting the Alertmanager architecture.
Description check	✅ Passed	The description follows the template closely, with a summary, checklist, validation, and release-notes section.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[SoloJacobs]

Hi @locker95 ,

This is the kind description that tends to go stale over time. I think it is probably better to have a link pointing towards architecture documentation, and put the description there (if it adds something).

Kind regards

[locker95]

Thanks @SoloJacobs, addressed in bf55531. The README now keeps only a short, stable summary and links to the architecture overview in docs/alertmanager.md. The overview links to the existing high-availability documentation for cluster-specific behavior, avoiding duplicated implementation details in the README.