Create protocol negotiation (multistream) doc

[yusefnapora]

We should have a concept doc about protocol negotiation that describes the high-level need, with details about how multistream is used throughout the libp2p stack.

We should emphasize that multistream is not directly related to multiplexing, but note that multiplexed streams do use multistream to identify the protocol used by each "inner stream".

Related concepts:

• transport
• stream multiplexing

[p-shahi]

Two of the 3 places where multistream is used will be changed. See:

• https://github.com/libp2p/specs/issues/426
• https://github.com/libp2p/specs/pull/353

And multistream will only remain for protocol negotiation (until it is also replaced by Protocol Select.) Therefore, setting this as lower priority for now.