Add .spi.yml for Swift Package Index DocC support

[DevYeom]

Related Issues 💭

Description 📝

This is a manifest file for the Swift Package Index to auto-generate and host DocC documentation.

Additional Notes 📚

Host DocC documentation in the Swift Package Index

Checklist ✅

• [x] If it's a new feature, have appropriate unit tests been added?
• [x] If the changes affect existing functionality, please verify whether the above information has been appropriately described.