feat(crons): Add missing .NET docs (#15697)

- Adds upsert docs
- Adds Heartbeat docs

Author: evanpurkhiser

platform-includes/crons/setup/dotnet.mdx

@@ -18,3 +18,88 @@ If your job execution fails, you can notify Sentry about the failure:
 // 🔴 Notify Sentry your job has failed:
 SentrySdk.CaptureCheckIn("<monitor-slug>", CheckInStatus.Error, checkInId);
 ```
+
+## Heartbeat
+
+Heartbeat monitoring notifies Sentry of a job's status through one check-in. This setup will only notify you if your job didn't run when expected (missed). If you want to monitor job timeouts, use the `InProgress` check-ins as indicated in the previous section.
+
+```csharp
+// Execute your scheduled task...
+
+// 🟢 Notify Sentry your job completed successfully:
+SentrySdk.CaptureCheckIn(
+    "<monitor-slug>",
+    CheckInStatus.Ok,
+    // Optionally annotate the check-in with the job duration
+    duration: TimeSpan.FromSeconds(10)
+);
+```
+
+If your job execution fails, you can:
+
+```csharp
+// 🔴 Notify Sentry your job has failed:
+SentrySdk.CaptureCheckIn(
+    "<monitor-slug>",
+    CheckInStatus.Error,
+    duration: TimeSpan.FromSeconds(10)
+);
+```
+
+## Upserting Cron Monitors
+
+You can create and update your Monitors programmatically with code
+rather than [creating and configuring them in Sentry.io](https://sentry.io/issues/alerts/new/crons/).
+
+```csharp
+// 🟡 Notify Sentry your job is running:
+var checkInId = SentrySdk.CaptureCheckIn(
+    "<monitor-slug>",
+    CheckInStatus.InProgress,
+    configureMonitorOptions: options =>
+    {
+        // Use a crontab schedule (every 10 minutes)
+        options.Interval("*/10 * * * *");
+        options.CheckInMargin = TimeSpan.FromMinutes(5);
+        options.MaxRuntime = TimeSpan.FromMinutes(15);
+        options.TimeZone = "Europe/Vienna";
+        options.FailureIssueThreshold = 2;
+        options.RecoveryThreshold = 5;
+    });
+
+// Execute your scheduled task here...
+
+// 🟢 Notify Sentry your job has completed successfully:
+SentrySdk.CaptureCheckIn("<monitor-slug>", CheckInStatus.Ok, checkInId);
+```
+
+Alternatively, you can use an interval schedule:
+
+```csharp
+// 🟡 Notify Sentry your job is running:
+var checkInId = SentrySdk.CaptureCheckIn(
+    "<monitor-slug>",
+    CheckInStatus.InProgress,
+    configureMonitorOptions: options =>
+    {
+        // Use an interval schedule (every 10 minutes)
+        options.Interval(10, SentryMonitorInterval.Minute);
+        options.CheckInMargin = TimeSpan.FromMinutes(5);
+        options.MaxRuntime = TimeSpan.FromMinutes(15);
+        options.TimeZone = "Europe/Vienna";
+    });
+
+// Execute your scheduled task here...
+
+// 🟢 Notify Sentry your job has completed successfully:
+SentrySdk.CaptureCheckIn("<monitor-slug>", CheckInStatus.Ok, checkInId);
+```
+
+Supported units are:
+
+- `SentryMonitorInterval.Minute`
+- `SentryMonitorInterval.Hour`
+- `SentryMonitorInterval.Day`
+- `SentryMonitorInterval.Week`
+- `SentryMonitorInterval.Month`
+- `SentryMonitorInterval.Year`