Add Intial DocC docs

[0xTim]

Adds some additional documentation. I don't know how far you want me to go with documenting everything so let me know. This should at least get the current APIs rendered on SPI. As an aside I'm not a huge fan of linking out to the CryptoKit docs since not everything is available here, all the code is already documented here and most of the tutorials and articles on developer.apple.com for CryptoKit refer to Apple Platform specific stuff, like interacting with the keychain, that is not applicable here.

Resolves #273

Checklist

• I've run tests to see all new and existing tests pass
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the documentation if necessary

If you've made changes to gyb files

• I've run .script/generate_boilerplate_files_with_gyb and included updated generated files in a commit of this pull request

Motivation:

Currently Swift Crypto has no documentation which is not great

Modifications:

Adds additional DocC scaffolding to take advantage of the DocC comments in Crypto and CryptoExtras. Adds an SPI config file for Swift Package Index

Result:

We will have some documentation