[docs]: add volume prune to command reference (#910)

## Type of Change
- [ ] Bug fix
- [ ] New feature  
- [ ] Breaking change
- [x] Documentation update

## Motivation and Context

Looks like this got accidentally deleted in #805

## Testing
- [ ] Tested locally
- [ ] Added/updated tests
- [x] Added/updated docs

Author: saehejkang

docs/command-reference.md

@@ -783,6 +783,20 @@ container volume delete vol1 vol2 vol3
 container volume delete --all
 ```
 
+### `container volume prune`
+
+Removes all volumes that have no container references. This includes volumes that are not attached to any running or stopped containers. The command reports the actual disk space reclaimed after deletion.
+
+**Usage**
+
+```bash
+container volume prune [--debug]
+```
+
+**Options**
+
+No options.
+
 ### `container volume list (ls)`
 
 Lists volumes.