feat(navigation): Keep top level nav consistent

What does this MR do?

Mention that parent nav items should not be replaced by a sub nav item.

This addition was inspired by the following issues/MRs talking about replacing CI/CD or Infrastructure with a sub nav item.

• Replacing "Infrastructure" with "Kubernetes" gitlab-org/gitlab#334011 (comment 635907733)
• Replacing "CI/CD" with "Runners" gitlab-org/gitlab!66376 (comment 633698780)

Does this MR meet the acceptance criteria?

• The MR title and commit message meet the Pajamas commit conventions
• Add a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
• Include MR scope that reflects the purpose of the change in the description
  • For example, if this MR is focused on usage guidelines, addressing accessibility challenges could be added separately with a different MR
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