plasmashell: add page (#13442)

* Create plasmashell.md

* Update pages/linux/plasmashell.md

Co-authored-by: spageektti <git@spageektti.cc>

---------

Co-authored-by: spageektti <git@spageektti.cc>

pages/linux/plasmashell.md

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+# plasmashell
+
+> Start and restart Plasma Desktop.
+> More information: <https://invent.kde.org/plasma/plasma-desktop>.
+
+- Restart `plasmashell`:
+
+`systemctl restart --user plasma-plasmashell`
+
+- Restart `plasmashell` without systemd:
+
+`plasmashell --replace & disown`
+
+- Display [h]elp on command-line options:
+
+`plasmashell --help`
+
+- Display help, including Qt options:
+
+`plasmashell --help-all`