Add .spi.yml for Swift Package Index docs

[scannillo]

Summary

Braintree iOS is currently accessible via the Swift Package Index (https://swiftpackageindex.com/braintree/braintree_ios). This PR enhances our Swift Package Index experience by supporting it's auto-hosting DocC feature.

Changes

• Add a .spi.yml file
  • See SPI docs on how this auto-hosted, auto-generated docs works. It's really neat! We could even have this replace our hosted reference docs entirely 👀.

Result

Once merged into master our SPI landing site will add this button:

The hosted docs & new button won't be visible until merged.

Checklist

• Added a changelog entry

Authors

@scannillo

[scannillo]

There seems to be a limitation with DocC supporting SDKs with multiple target frameworks (see Issue - swiftlang/swift-docc#255). That issue also links a Swift Forums discussion on the topic.

I don't mind merging, seeing how SPI handles this, and then reverting if its borked.

[adam-rocska]

@scannillo did it work? Just saw your comment it’ll get reverted if it doesn’t work, and kept if it works ¯\(ツ)/¯

[scannillo]

Hi @adam-rocska, this did indeed work! You can navigate here (https://swiftpackageindex.com/braintree/braintree_ios) and click Documentation to see the docs that SPI is building on our behalf.

We would love feedback as well to your experience using those docs versus our self-hosted reference docs.

[adam-rocska]

@scannillo Hl s*t! THANKS! ❤️