Add GravatarUI docs to the SPI documentation building

[andrewdmontgomery]

Closes #

Description

This adds the GravatarUI docs back in to the SPI documentation building job. It was originally removed in #302 as a test.

Testing Steps

I believe the only way to test this is to merge it into a branch. We can merge it into a rc branch/tag though before releasing in the final release.

[wpmobilebot]

📲 You can test the changes from this Pull Request in Gravatar SwiftUI Prototype Build by scanning the QR code below to install the corresponding build.

	App Name	Gravatar SwiftUI Prototype Build
	Build Number	1511
	Version	1.0
	Bundle ID	com.automattic.gravatar-sdk-demo-swiftui.prototype-build
	Commit	90e1063bbdd6dec836dcfe83ae1b9de25f05cbfe
	App Center Build	Gravatar SDK Demo - SwiftUI #276

Automatticians: You can use our internal self-serve MC tool to give yourself access to App Center if needed.

[wpmobilebot]

📲 You can test the changes from this Pull Request in Gravatar UIKit Prototype Build by scanning the QR code below to install the corresponding build.

	App Name	Gravatar UIKit Prototype Build
	Build Number	1511
	Version	1.0
	Bundle ID	com.automattic.gravatar-sdk-demo-uikit.prototype-build
	Commit	90e1063bbdd6dec836dcfe83ae1b9de25f05cbfe
	App Center Build	Gravatar SDK Demo - UIKit #277

Automatticians: You can use our internal self-serve MC tool to give yourself access to App Center if needed.