Add a note about not restricting CI component keywords yet

Grzegorz Bizonrequested to merge
docs/gb/restricting-ci-component-keywords-note into master

What does this MR do and why?

This MR clarifies why we decided not to restrict CI Components' top-level keywords in the first iteration of the CI Catalog.

/cc @fabiopitino @dhershkovitch @ayufan

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