lsfd: add page (#15284)

Co-authored-by: Juri Dispan <juri.dispan@posteo.net>
Co-authored-by: K.B.Dharun Krishna <kbdharunkrishna@gmail.com>

pages/linux/lsfd.md

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+# lsfd
+
+> List open files and the corresponding processes in Linux.
+> More information: <https://manned.org/lsfd>.
+
+- List all open file descriptors:
+
+`lsfd`
+
+- List all files kept open by a specific program:
+
+`lsfd -Q 'PID == {{process_ID}}'`
+
+- Check what program has a specific file open:
+
+`lsfd -Q "NAME == '{{/path/to/file}}'"`
+
+- List open IPv4 or IPv6 sockets:
+
+`lsfd -i{{4|6}}`