systemd-sysext: add page (#10724)

* systemd-sysext: add page

---------

Co-authored-by: K.B.Dharun Krishna <kbdharunkrishna@gmail.com>
Co-authored-by: Lena <126529524+acuteenvy@users.noreply.github.com>

pages/linux/systemd-sysext.md

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+# systemd-sysext
+
+> Activate or deactivate system extension images.
+> More information: <https://www.freedesktop.org/software/systemd/man/systemd-sysext.html>.
+
+- List installed extension images:
+
+`systemd-sysext list`
+
+- Merge system extension images into `/usr/` and `/opt/`:
+
+`systemd-sysext merge`
+
+- Check the current merge status:
+
+`systemd-sysext status`
+
+- Unmerge all currently installed system extension images from `/usr/` and `/opt/`:
+
+`systemd-sysext unmerge`
+
+- Refresh system extension images (a combination of unmerge and merge):
+
+`systemd-sysext refresh`