Remove a reference to an unused page from README.md

[irfano]

Description

This removes the "Subtree'd Library Projects" link from the docs list on the README page. The page was already removed with #5407 since subtree library projects are unused.

Testing information

Review the README.md to ensure the link to "Subtree'd Library Projects" has been removed.

• [x] I have considered if this change warrants release notes and have added them to RELEASE-NOTES.txt if necessary. Use the "[Internal]" label for non-user-facing changes.

[wpmobilebot]

📲 You can test the changes from this Pull Request in WooCommerce-Wear Android by scanning the QR code below to install the corresponding build.

	App Name	WooCommerce-Wear Android
	Platform	⌚️ Wear OS
	Flavor	Jalapeno
	Build Type	Debug
	Commit	8f8b0c6498363af5c1604494dd5093ce4d75579a
	Direct Download	woocommerce-wear-prototype-build-pr11856-8f8b0c6.apk

[wpmobilebot]

📲 You can test the changes from this Pull Request in WooCommerce Android by scanning the QR code below to install the corresponding build.

	App Name	WooCommerce Android
	Platform	📱 Mobile
	Flavor	Jalapeno
	Build Type	Debug
	Commit	8f8b0c6498363af5c1604494dd5093ce4d75579a
	Direct Download	woocommerce-prototype-build-pr11856-8f8b0c6.apk