Clarify the container registry image naming convention

🐼 Context

We have a confusion on this section https://docs.gitlab.com/ee/user/packages/container_registry/#image-naming-convention.

If we look at the examples:

• We can use exactly the project full path as an image path.
• We can only add two additional names at the project full path, not three.

This MR clarifies those two points.

🔬 What does this MR do?

• Update the Image naming convention section in the container registry documentation.

⛓ Related issues

#378005 (closed)

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