Add Intial DocC docs

.spi.yml

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+version: 1
+builder:
+  configs:
+    - documentation_targets: [Crypto, _CryptoExtras]

Sources/Crypto/Docs.docc/index.md

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+# ``Crypto``
+
+A cryptography library for Swift.
+
+## Overview
+
+Swift Crypto provides a Swift library for common cryptographic operations. It is available as a Swift package and provides two main libraries:
+
+* `Crypto` - an open-source implementation of a substantial portion of the API of [Apple CryptoKit](https://developer.apple.com/documentation/cryptokit) suitable for use on Linux platforms. It enables cross-platform or server applications with the advantages of CryptoKit.
+* `CryptoExtras` - a collection of additional cryptographic primitives and utilities that are not part of CryptoKit but useful in a server environment.
+
+Swift Crypto is built on top of [BoringSSL](https://boringssl.googlesource.com/boringssl/), Google's fork of OpenSSL. The current features of Swift Crypto cover key exchange, key derivation, encryption and decryption, hashing, message authentication, and more.

Sources/_CryptoExtras/Docs.docc/index.md

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+# ``CryptoExtras``
+
+Provides additional cryptographic APIs that are not available in CryptoKit (and therefore the core Crypto library).
+
+