docs: mention new create backup api in docs and changelog (#7104)

### Describe Your Changes

Please provide a brief description of the changes you made. Be as
specific as possible to help others understand the purpose and impact of
your modifications.

### Checklist

The following checks are **mandatory**:

- [x] My change adheres [VictoriaMetrics contributing
guidelines](https://docs.victoriametrics.com/contributing/).

Signed-off-by: Artem Navoiev <tenmozes@gmail.com>

docs/changelog/CHANGELOG.md

@@ -31,7 +31,8 @@ See also [LTS releases](https://docs.victoriametrics.com/lts-releases/).
 * FEATURE: [dashboards](https://github.com/VictoriaMetrics/VictoriaMetrics/blob/master/dashboards) for VM single-node, cluster, vmalert, vmagent, VictoriaLogs: add `Go scheduling latency` panel to show the 99th quantile of Go goroutines scheduling. This panel should help identifying insufficient CPU resources for the service. It is especially useful if CPU gets throttled, which now should be visible on this panel.
 * FEATURE: [alerts](https://github.com/VictoriaMetrics/VictoriaMetrics/blob/master/deployment/docker/alerts-health.yml): add alerting rule to track the Go scheduling latency for goroutines. It should notify users if VM component doesn't have enough CPU to run or gets throttled.
 * FEATURE: [vmagent](https://docs.victoriametrics.com/vmagent/) and [Single-node VictoriaMetrics](https://docs.victoriametrics.com/): hide jobs that contain only healthy targets when `show_only_unhealthy` filter is enabled. Before, jobs without unhealthy targets were still displayed on the page. See [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/3536).
-* FEATURE: [vmalert](https://docs.victoriametrics.com/vmalert): bump default values for sending data to `remoteWrite.url`: `remoteWrite.maxQueueSize` from `100_000` to `1_000_000`, `remoteWrite.maxBatchSize` from `1_000` to `10_000`, `remoteWrite.concurrency` from `1` to `4`. The new settings should improve remote write performance of vmalert with default settings.  
+* FEATURE: [vmalert](https://docs.victoriametrics.com/vmalert): bump default values for sending data to `remoteWrite.url`: `remoteWrite.maxQueueSize` from `100_000` to `1_000_000`, `remoteWrite.maxBatchSize` from `1_000` to `10_000`, `remoteWrite.concurrency` from `1` to `4`. The new settings should improve remote write performance of vmalert with default settings.
+* FEATURE: [vmbackupmanager](https://docs.victoriametrics.com/vmbackupamanager/): add API for creating/scheduling backups. See [documentation](https://docs.victoriametrics.com/vmbackupmanager/#api-methods)
 
 * BUGFIX: [vmagent](https://docs.victoriametrics.com/vmagent/) fix service discovery of Azure Virtual Machines for response contains `nextLink` in `Host:Port` format. See [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/6912).
 * BUGFIX: [vmagent](https://docs.victoriametrics.com/vmagent/): properly consume messages [from kafka](https://docs.victoriametrics.com/vmagent/#kafka-integration). Previously vmagent could skip some messages during start-up.

docs/vmbackupmanager.md

@@ -202,19 +202,25 @@ For example:
 
 `vmbackupmanager` exposes the following API methods:
 
+* POST `/api/v1/backups` - schedule/create the backup. Response examples:
+
+  success, status code - 201, body:
+  ```json
+      {}
+  ```
+
+  failure, status code - 400, body:
+  ```json
+      {"error": "backups <name> is in progress"}
+  ```
+
 * GET `/api/v1/backups` - returns list of backups in remote storage.
-  Example output:
+  Response example:
   ```json
   [{"name":"daily/2023-04-07","size_bytes":318837,"size":"311.4ki","created_at":"2023-04-07T16:15:07+00:00"},{"name":"hourly/2023-04-07:11","size_bytes":318837,"size":"311.4ki","created_at":"2023-04-07T16:15:06+00:00"},{"name":"latest","size_bytes":318837,"size":"311.4ki","created_at":"2023-04-07T16:15:04+00:00"},{"name":"monthly/2023-04","size_bytes":318837,"size":"311.4ki","created_at":"2023-04-07T16:15:10+00:00"},{"name":"weekly/2023-14","size_bytes":318837,"size":"311.4ki","created_at":"2023-04-07T16:15:09+00:00"}]
   ```
   > Note: `created_at` field is in RFC3339 format.
-
-* GET `/api/v1/backups/<BACKUP_NAME>` - returns backup info by name.
-  Example output:
-  ```json
-  {"name":"daily/2023-04-07","size_bytes":318837,"size":"311.4ki","created_at":"2023-04-07T16:15:07+00:00","locked":true}
-  ```
-
+  
 * PUT `/api/v1/backups/<BACKUP_NAME>` - update "locked" attribute for backup by name.
   Example request body:
   ```json