Add package plugin instructions to documentation

[franklinsch]

Bug/issue #, if applicable: N/A

Summary

Add instructions to set up the plugin in the documentation. When reading the documentation at https://apple.github.io/swift-docc-plugin/documentation/swiftdoccplugin/, it might not be totally clear that the plugin needs to be added to your package first, especially if the developer is not yet familiar with the concept of SwiftPM plugins.

Dependencies

None.

Testing

No functional changes.

Checklist

• ~[ ] Added tests~ N/A
• [x] Ran the ./bin/test script and it succeeded
• [x] Updated documentation if necessary

[franklinsch]

@swift-ci please test

[ethan-kusters]

@swift-ci please test