Enable Swift Package Index documentation and add API doc link

[Kyle-Ye]

Bug/issue #, if applicable: None

Summary

1. Add .spi.yml file to make SwiftPackageIndex build documentation for this package.
2. Update README.md to add a link pointing to the corresponding link. (The link will be available on the next day after the PR is merged. SPI's CI is triggered with a 24h window.)

See https://github.com/apple/swift-algorithms/blob/main/.spi.yml

Dependencies

None

Testing

None

Checklist

• Ran the swift test and it succeeded
• Updated documentation if necessary

Future work

After this PR, we'll only get the basic docc documentation support.

• Add SystemPackage.docc in Sources/System to adjust the doc page.

[Kyle-Ye]

@swift-ci please test

[Kyle-Ye]

@swift-ci please test

[MaxDesiatov]

@Kyle-Ye would you kindly update the description providing more context and motivation for the change? Thanks!

[Kyle-Ye]

@Kyle-Ye would you kindly update the description providing more context and motivation for the change? Thanks!

Done. (There is no PR template in this repo so I forgot to add description here. Sorry for that. And I use the template in swift-docc to fill it in the end.)

Do you think we should address the "Future work" part in this PR?

[MaxDesiatov]

IMO no need to get the PR bigger than it is now, let's wait for feedback from maintainers first.