feat(ContextualHelp): Clarify external link icon

What does this MR do?

Add explicit guidance to https://design.gitlab.com/usability/contextual-help#external-links not to add the external link icon when linking docs.

As it is now, a SaaS contributor might think that, because the docs link opens a page on another site (docs.gitlab.com), this qualifies as a link "outside of the GitLab instance".

Related: gitlab-org/gitlab!123581 (comment 1517427170)

Does this MR meet the acceptance criteria?

• The MR title and commit messages meet the Pajamas commit conventions.
• The “What does this MR do?” section in the MR description is filled out, explaining the reasons for and scope of the proposed changes, per “Say why not just what”.
  • For example, if the MR is focused on usage guidelines, addressing accessibility challenges could be added in a separate MR.
• Relevant label(s) are applied to the MR.
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• Review requested from any GitLab designer or directly from a maintainer or trainee maintainer.