Improve documentation

alerting/provider/provider.go

@@ -5,7 +5,11 @@ import (
 	"github.com/TwinProduction/gatus/core"
 )
 
+// AlertProvider is the interface that each providers should implement
 type AlertProvider interface {
+	// IsValid returns whether the provider's configuration is valid
 	IsValid() bool
+
+	// ToCustomAlertProvider converts the provider into a custom.AlertProvider
 	ToCustomAlertProvider(service *core.Service, alert *core.Alert, result *core.Result, resolved bool) *custom.AlertProvider
 }