Composable Image: Better explain the most common usecases for using Composable Image

[thomasdiluccio]

Where on docs.platform.sh should be changed?

• https://docs.upsun.com/create-apps/app-reference/composable-image.html
• https://docs.upsun.com/create-apps/app-reference.html
• probably also every place else where we mention Single-Runtime Image vs Composable

What exactly should be updated?

New users might be puzzled by this choice: "Single-runtime Image" vs "Composable Image". We should reduce the ambiguity and complexity by saying that users most likely want to follow the single-runtime route. But, on some occasion, it is better to user Composable Image:

• Running a static site
• Installing binaries (from the list of requested binaries from customers)
• Changing the secondary runtime version (such as PHP 8.4 and NodeJS 22)

Additional context

No response