docs: add checker reference pages and update homepage feature list

Add individual reference pages for all domain health checkers (EN/FR),
update the homepage feature descriptions in both languages to highlight
monitoring, notifications, and domain availability checks.

content/reference/checkers/_index.en.md

@@ -0,0 +1,36 @@
+---
+date: 2026-06-11T09:00:00+02:00
+title: Checkers
+author: nemunaire
+archetype: chapter
+weight: 35
+aliases:
+    checkers
+---
+
+Checkers are the building blocks of happyDomain's monitoring system. Each one collects data about a domain, a zone or a service, evaluates it against a set of rules, and reports a clear status (OK, Warning, Critical or Error).
+
+This chapter documents every checker shipped with happyDomain: what it verifies, the scope it applies to, the options you can tune, and the rules it evaluates. For the day-to-day workflow of configuring, scheduling and reading checks in the interface, see {{< relref "/pages/checks" >}}.
+
+## Scopes
+
+Every checker declares the scope it operates on:
+
+- **Domain-level** — concerns the domain itself, independent of its DNS records (registration status, expiry, transfer lock…).
+- **Zone-level** — needs the full zone content (DNSSEC validation, delegation consistency…).
+- **Service-level** — targets a specific service published on a subdomain (HTTP, TLS, ping…), and is configured from that service's own **Checks** tab.
+
+## Statuses
+
+Checkers report one of the following statuses, in order of severity:
+
+| Status | Meaning |
+|--------|---------|
+| **OK** | Everything is within acceptable parameters |
+| **Info** | Informational finding, no action needed |
+| **Warning** | A threshold is approaching; attention recommended |
+| **Critical** | A threshold has been exceeded; action required |
+| **Error** | The check itself failed (collection error, bad configuration) |
+| **Unknown** | The check could not determine a result |
+
+{{% children %}}