Write concept doc on peers and peer IDs

[Mr0grog]

This issue is part of Epic 3B: Fixes from legacy issue queue.

At the Berlin developer summit, we decided Peers and Peer IDs were important to write a concept doc about.

These are tightly related enough that we could probably just have one doc covering both and name it “Peers and Peer IDs,” “Peer/Peer ID,” “Peer(ID),” or something along those lines.

We did not have time to write a rough draft, so this needs to be written from scratch.

This is a subtask of #56.

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Concept docs are concise, high-level descriptions of various core concepts in IPFS. They live in the content/guides/concepts folder and should strive to answer:

1. What is this?
2. How does it relate to the rest of IPFS?
3. How can (or should?) you use it? (Note: sometimes the answer is that you should understand it, but not use it. Bitswap is probably a good example.)
4. Where do you go to learn more?
5. What is the current state of affairs?

[jessicaschilling]

libp2p has addressed this - see libp2p/docs#35 .

[jessicaschilling]

Closing this issue, since it's been addressed by the libp2p folks (https://docs.libp2p.io/concepts/peer-id/) and we're including some form of a basic libp2p guide in the docs beta: cf https://github.com/ipfs/ipfs-docs-v2