doc: UPNP: Add example on how to open/close a port
doc: UPNP: Add example on how to open/close a port

I have added a small code sample to show how to automate the process of port forwarding (and closing after you are done).
Thanks to @mhilbrunner for explaining how to achieve t0his (see #18780)

guilhermefelipecgs/godot,MarianoGnu/godot,honix/godot,MarianoGnu/godot,firefly2442/godot,sanikoyes/godot,Shockblast/godot,BastiaanOlij/godot,DmitriySalnikov/godot,ZuBsPaCe/godot,sanikoyes/godot,godotengine/godot,DmitriySalnikov/godot,pkowal1982/godot,vkbsb/godot,honix/godot,Paulloz/godot,DmitriySalnikov/godot,pkowal1982/godot,BastiaanOlij/godot,Zylann/godot,Paulloz/godot,Shockblast/godot,vkbsb/godot,godotengine/godot,ZuBsPaCe/godot,josempans/godot,guilhermefelipecgs/godot,akien-mga/godot,godotengine/godot,ZuBsPaCe/godot,Faless/godot,Paulloz/godot,Zylann/godot,Shockblast/godot,DmitriySalnikov/godot,ex/godot,Faless/godot,josempans/godot,akien-mga/godot,firefly2442/godot,firefly2442/godot,ZuBsPaCe/godot,DmitriySalnikov/godot,akien-mga/godot,ex/godot,akien-mga/godot,sanikoyes/godot,ZuBsPaCe/godot,honix/godot,Shockblast/godot,Zylann/godot,godotengine/godot,Zylann/godot,sanikoyes/godot,vnen/godot,DmitriySalnikov/godot,Valentactive/godot,Paulloz/godot,josempans/godot,BastiaanOlij/godot,akien-mga/godot,vkbsb/godot,ex/godot,sanikoyes/godot,godotengine/godot,vnen/godot,vkbsb/godot,Shockblast/godot,vnen/godot,firefly2442/godot,vnen/godot,guilhermefelipecgs/godot,vkbsb/godot,vkbsb/godot,MarianoGnu/godot,Paulloz/godot,BastiaanOlij/godot,BastiaanOlij/godot,Valentactive/godot,pkowal1982/godot,Shockblast/godot,Valentactive/godot,Paulloz/godot,Faless/godot,guilhermefelipecgs/godot,Faless/godot,Paulloz/godot,honix/godot,firefly2442/godot,Faless/godot,ex/godot,ex/godot,Zylann/godot,josempans/godot,pkowal1982/godot,Zylann/godot,guilhermefelipecgs/godot,Valentactive/godot,firefly2442/godot,guilhermefelipecgs/godot,MarianoGnu/godot,Faless/godot,Shockblast/godot,MarianoGnu/godot,vnen/godot,godotengine/godot,BastiaanOlij/godot,josempans/godot,ZuBsPaCe/godot,Zylann/godot,Valentactive/godot,Zylann/godot,DmitriySalnikov/godot,akien-mga/godot,MarianoGnu/godot,sanikoyes/godot,vkbsb/godot,pkowal1982/godot,BastiaanOlij/godot,ex/godot,godotengine/godot,sanikoyes/godot,pkowal1982/godot,akien-mga/godot,sanikoyes/godot,josempans/godot,vkbsb/godot,pkowal1982/godot,honix/godot,guilhermefelipecgs/godot,josempans/godot,Valentactive/godot,vnen/godot,ex/godot,ex/godot,MarianoGnu/godot,vnen/godot,Shockblast/godot,guilhermefelipecgs/godot,firefly2442/godot,godotengine/godot,Faless/godot,Valentactive/godot,Valentactive/godot,akien-mga/godot,firefly2442/godot,pkowal1982/godot,BastiaanOlij/godot,ZuBsPaCe/godot,vnen/godot,Faless/godot,ZuBsPaCe/godot,MarianoGnu/godot,josempans/godot,honix/godot