docs: start docs mdbook

docs/src/SUMMARY.md

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+# Summary
+
+- [Overview](./overview.md)
+- [Concepts](./concepts/README.md)
+    - [Root key](./concepts/rootkey.md)
+- [Specifications](./spec/README.md)
+    - [Authn](./spec/authn.md)

docs/src/concepts.md

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+# Concepts

docs/src/concepts/README.md

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+# Concepts
+
+- [Root key](./rootkey.md)

docs/src/concepts/rootkey.md

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+# Root key
+
+A Root key is an Ed25519 public key, that represents the root of an identity in Pubky.

docs/src/overview.md

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+# Overview

docs/src/spec/README.md

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+# Specifications
+
+Detail specifications of various aspects of the protocol.
+
+- [Authn](./authn.md)

docs/src/spec/authn.md

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+# Pubky Authn
+
+Pubky Authn is a simple protocol for using user's [root key](../concepts/rootkey.md),
+to authenticate themselves to a service provider.
+
+## How it works 
+
+A client with access to the user's root key will begin by generating a time step and a nonce.