Move/update relationships content #4773
Description
Where on docs.platform.sh should be changed?
https://docs.platform.sh/create-apps/app-reference/single-runtime-image.html#relationships
https://docs.upsun.com/create-apps/app-reference/single-runtime-image.html#relationships
What exactly should be updated?
This step is part of the overall effort to streamline the single/composable image content.
Relationships content is common to both single and composable images. This PR:
- Keeps the most important relationships content in the single/composable topics
- Moves the remaining common/conceptual resources content into its own topic, which is referenced by both the single/composable topic
Additional context
No response