Add warning that components cannot use include keyword

The page mentions that components should be self-contained but doesn't explicitly state that the include keyword is not allowed in components.

Location https://docs.gitlab.com/ee/ci/components/

Current Text "You cannot use spec:include in components. Components should be self-contained and not depend on external files. Define inputs directly in the component instead of including them from separate files."

Suggested Fix Add a warning at the beginning of the "Write a component" section: "⚠️ IMPORTANT: Components cannot use the include keyword. All configuration must be defined directly in the component file. If you need reusable configuration, use YAML anchors or the extends keyword instead."

Why This Helps Users don't waste time trying to use include in components only to find it doesn't work.

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