Add dotnet to the implementations page

[dhuseby]

From @flcl42:

.NET libp2p has the following working parts(and I hope the list will grow in the next couple of months):

libp2p-implementations (In Progress / Usable) https://github.com/NethermindEth/dotnet-libp2p

libp2p-tcp (In Progress / Usable) https://github.com/NethermindEth/dotnet-libp2p
libp2p-quic (Prototype / Unstable) https://github.com/NethermindEth/dotnet-libp2p/tree/main/src/libp2p/Libp2p.Protocols.Quic

libp2p-noise (In Progress / Usable) https://github.com/NethermindEth/dotnet-libp2p/tree/main/src/libp2p/Libp2p.Protocols.Noise

mdns-discovery (In Progress / Usable) https://github.com/NethermindEth/dotnet-libp2p

libp2p-floodsub (In Progress / Usable) https://github.com/NethermindEth/dotnet-libp2p
libp2p-gossipsub (In Progress / Usable) https://github.com/NethermindEth/dotnet-libp2p

libp2p-yamux (In Progress / Usable) https://github.com/NethermindEth/dotnet-libp2p

libp2p-ping (In Progress / Usable) https://github.com/NethermindEth/dotnet-libp2p