Migrate documentation for Package Registry Server Specification to DocC

[bripeticca]

Description

In an effort to create a consolidated single source for the swift package manager documentation, this issue will track the work to be done in moving the contents of PackageRegistry/Registry.md into our DocC catalogue.

Expected behavior

No response

Actual behavior

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Steps to reproduce

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Swift Package Manager version/commit hash

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Swift & OS version (output of swift --version && uname -a)

No response