userdbctl: add page (#10662)

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

pages/linux/userdbctl.md

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+# userdbctl
+
+> Inspect users, groups and group memberships on the system.
+> More information: <https://www.freedesktop.org/software/systemd/man/userdbctl.html>.
+
+- List all known user records:
+
+`userdbctl user`
+
+- Show details of a specific user:
+
+`userdbctl user {{username}}`
+
+- List all known groups:
+
+`userdbctl group`
+
+- Show details of a specific group:
+
+`userdbctl group {{groupname}}`
+
+- List all services currently providing user/group definitions to the system:
+
+`userdbctl services`