Include a functional example in GraphQL docs

What does this MR do?

Intent: Set up a functional template to help our readers actually use our GraphQL implementation to get data. To do so:

• Set up dedicated pages for each example (sometimes known as a "use case")
• Start each new page with an explanation
  • Include each new page in the doc/api/graphql directory
• Include a "working" GraphQL code block that can be copied to the GraphiQL Explorer
  • Include graphql as a label for the code block
• Include a screenshot of the GraphiQL explorer, with the YAML example and output, so readers know what to expect
  • Include each screenshot in the doc/api/graphql/img directory
• Set up the docs.gitlab.com/ee ToC to include the title of each dedicated page, "side by side" with the GraphQL reference
• Refer to the reference page for properties/attributes. Given the principles of SSoT, it should not be necessary to define any attributes on the dedicated example page

If approved, the logical follow-up would be to set up an MR template dedicated to GraphQL examples.

The functional doc example included in this MR is based on the linked issue, ref: #215661

I set up this template based on GitLab Contribute 2020 discussions.

Related issues

Author's checklist (required)

• Follow the Documentation Guidelines and Style Guide.
• If you have developer access or higher (for example, GitLab team members or Core Team members)
  • Apply the documentation label, plus:
    • The corresponding DevOps stage and group label, if applicable.
    • development guidelines when changing docs under doc/development/*, CONTRIBUTING.md, or README.md.
    • development guidelines and Documentation guidelines when changing docs under development/documentation/*.
    • development guidelines and Description templates (.gitlab/*) when creating/updating issue and MR description templates.
  • Assign the designated Technical Writer.

When applicable:

• Update the permissions table.
• Link docs to and from the higher-level index page, plus other related docs where helpful.
• Add GitLab's version history note(s).
• Add the product tier badge.
• Add/update the feature flag section.
• If you're changing document headings, search doc/*, app/views/*, and ee/app/views/* for old headings replacing with the new ones to avoid broken anchors.

Review checklist

All reviewers can help ensure accuracy, clarity, completeness, and adherence to the Documentation Guidelines and Style Guide.

1. Primary Reviewer

• Review by a code reviewer or other selected colleague to confirm accuracy, clarity, and completeness. This can be skipped for minor fixes without substantive content changes.

2. Technical Writer

• Optional: Technical writer review. If not requested for this MR, must be scheduled post-merge. To request for this MR, assign the writer listed for the applicable DevOps stage.
  • Add Technical Writing and docs:: workflow label.
  • Add docs-only when the only files changed are under doc/*.

3. Maintainer

1. Review by assigned maintainer, who can always request/require the above reviews. Maintainer's review can occur before or after a technical writer review.
2. Ensure a release milestone is set.
3. If there has not been a technical writer review, create an issue for one using the Doc Review template.