upnpc: add page (#14125)

Co-authored-by: K.B.Dharun Krishna <kbdharunkrishna@gmail.com>
Co-authored-by: Wiktor Perskawiec <git@spageektti.cc>
Co-authored-by: Sebastiaan Speck <12570668+sebastiaanspeck@users.noreply.github.com>

pages/linux/upnpc.md

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+# upnpc
+
+> Configure port forwarding rules on your router via the UPnP protocol.
+> More information: <https://manned.org/upnpc>.
+
+- Forward the external TCP port 80 to port 8080 on a local machine:
+
+`upnpc -a {{192.168.0.1}} 8080 80 tcp`
+
+- Delete any port redirection for external TCP port 80:
+
+`upnpc -d 80 tcp`
+
+- Get information about UPnP devices on your network:
+
+`upnpc -s`
+
+- List existing redirections:
+
+`upnpc -l`