add container size to the resources example

[matthiaz]

Clarify resource allocation details and suggest Upsun Flex Organization for advanced control.

Why

The example didn't explictly set a size. Which is a problem when you realize the resources key uses the vCPU from the container size. I keep recommending to customers to explicitly set it, after it breaks things. But it might be better to just have this correct in the docs.

What's changed

added size key in example.

Also made a slight change in the wording of the upsun flex advert.

Where are changes

Updates are for:

• platform (sites/platform templates)
• upsun (sites/upsun templates)

[github-actions]

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See the changed pages:

Upsun Fixed docs

• https://mr-matthias-resources-eo6chqy-ucq44jg6ofare.eu-5.platformsh.site/create-apps/flexible-resources.html

Upsun docs