Add note about specifying stages when using include

What does this MR do?

This MR updates the docs about stages to clarify what happens when you do specify stages.

We say:

If you do not define stages in the pipeline, you can use the 5 default stages

I'm adding a clarifying note about what happens if you do define stages. It may not be immediately clear to all that when you do define stages, the default stages are not added. This can lead to errors when you manually define stages and then include a template that attempts to add a job to a stage that you have not manually defined.

This is motivated by 🎫 a ticket that GitLab team members with access to ZenDesk can review for additional context.

We talk about manually adding a test stage when one is not declared in this troubleshooting section from the application security page but I think we should clarify it here where we talk about defining stages. People will encounter this outside of the CI templates we provide and so may not immediately draw a connection between what they are doing and the linked troubleshooting section.

Note: I am happy for any and all improvements to the wording I have proposed.

Screenshots (strongly suggested)

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