Move parent-child pipelines to downstream page

What does this MR do?

The final step combining the multi-project and parent-child project content together.

This MR moves parent-child content into the downstream pipelines page. Almost no edits, though I did:

• Move the "mirror the status of the downstream pipeline" example section in the parent and child pipelines page right into the already existing example in the downstream page (this kind of merging of information is the whole reason for the work). Tabs make this content a dream to present now!
• Rename the # Examples header to the clearer # Trigger a parent-child pipeline
• Delete the # Pass CI/CD variables to a child pipeline section, which was nothing but a list of crosslinks to this current page.

Review app: http://main-ee-96990.35.193.151.162.nip.io/ee/ci/pipelines/downstream_pipelines.html

Following this, we can update the navbar to match in gitlab-docs!3076 (merged)

Related issues

Related to #371483 (closed)

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
• Follow the:
  • Documentation process.
  • Documentation guidelines.
  • Style Guide.
• If you're adding or changing the main heading of the page (H1), ensure that the product tier badge is added.
• If you are a GitLab team member, request a review based on:
  • The documentation page's metadata.
  • The associated Technical Writer.

If you are a GitLab team member and only adding documentation, do not add any of the following labels:

• ~"frontend"
• ~"backend"
• ~"type::bug"
• ~"database"

These labels cause the MR to be added to code verification QA issues.

Reviewer's checklist

Documentation-related MRs should be reviewed by a Technical Writer for a non-blocking review, based on Documentation Guidelines and the Style Guide.

• If the content requires it, ensure the information is reviewed by a subject matter expert.
• Technical writer review items:
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  • Ensure a release milestone is set.
  • If relevant to this MR, ensure content topic type principles are in use, including:
    • The headings should be something you'd do a Google search for. Instead of Default behavior, say something like Default behavior when you close an issue.
    • The headings (other than the page title) should be active. Instead of Configuring GDK, say something like Configure GDK.
    • Any task steps should be written as a numbered list.
    • If the content still needs to be edited for topic types, you can create a follow-up issue with the docs-technical-debt label.
• Review by assigned maintainer, who can always request/require the reviews above. Maintainer's review can occur before or after a technical writer review.