synoupgrade: add page (#6262)

pages/linux/synoupgrade.md

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+# synoupgrade
+
+> Upgrade a Synology DiskStation Manager (DSM) from the command-line.
+> More information: <https://www.synology.com/dsm>.
+
+- Check if upgrades are available:
+
+`sudo synoupgrade --check`
+
+- Check for patches without upgrading the DSM version:
+
+`sudo synoupgrade --check-smallupdate`
+
+- Download the latest upgrade available (use `--download-smallupdate` for patches):
+
+`sudo synoupgrade --download`
+
+- Start the upgrade process:
+
+`sudo synoupgrade --start`
+
+- Upgrade to the latest version automatically:
+
+`sudo synoupgrade --auto`
+
+- Apply patches without upgrading the DSM version automatically:
+
+`sudo synoupgrade --auto-smallupdate`
+
+- Upgrade the DSM using a patch file (should be an absolute path):
+
+`sudo synoupgrade --patch {{/path/to/file.pat}}`
+
+- Display help:
+
+`synoupgrade`