Concept Doc: DHTs or Content Routing

[Mr0grog]

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

We should have a short explainer about either DHTs or content routing. It should contain:

• A brief overview of the general concept of DHTs (link to a good doc somewhere else that explains more)
• An explanation of how their role in IPFS (to find content, to store IPNS records)
• Mention that our DHT is Kademlia. This probably doesn’t need more detail beyond noting a few important high-level implications of that choice, if there are any. Link somewhere else for more details.

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?