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

@@ -32,6 +32,7 @@ See also [LTS releases](https://docs.victoriametrics.com/lts-releases/).
 * 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: [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