systemctl-list-unit-files: add page

pages/linux/systemctl-list-unit-files.md

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+# systemctl list-unit-files
+
+> List installed unit files and their enablement states.
+> See also: `systemctl list-units` for listing units currently loaded in memory.
+> More information: <https://www.freedesktop.org/software/systemd/man/latest/systemctl.html#list-unit-files%20PATTERN%E2%80%A6>.
+
+- List installed unit files and their enablement states:
+
+`systemctl list-unit-files`
+
+- Filter by state (e.g., enabled):
+
+`systemctl list-unit-files --state enabled`
+
+- Filter by a name pattern (e.g., sshd*):
+
+`systemctl list-unit-files 'sshd*'`
+
+- Print output directly to stdout (disable pager):
+
+`systemctl list-unit-files --no-pager`
+
+- Print output without headers or footers (for scripts):
+
+`systemctl list-unit-files --no-legend`