Detail the deprecation of mplex

[p-shahi]

Ref: libp2p/specs#553

libp2p implementations are in the process of deprecating mplex (in favor of yamux or transports with built in mplexing like QUIC) - this blog post should detail the motivations and describe historical reasons for supporting mplex this long

[marten-seemann]

Good idea! @p-shahi, could you draft something? Would be nice to get this blog post out pretty soon.