Add use cases to Iterations docs

What does this MR do?

Add use cases to https://docs.gitlab.com/ee/user/group/iterations/.

Use case 1: Sprint planning and execution

• Scenario: Product managers and development teams need to plan and execute work in fixed time periods to maintain a predictable delivery cadence. Iterations help structure this workflow by providing clear timeboxes and work tracking. Link to https://docs.gitlab.com/ee/tutorials/scrum_events/index.html.
• Task: Plan and execute a series of 2-week development sprints with multiple teams.
• How iterations are used: Create an iteration cadence for sprint timeboxes, automate sprint rollovers, group issues by iteration on boards, track progress through burndown charts.
• Benefit: Provides structure and automation for Scrum ceremonies while making progress visible to all stakeholders.

Use case 2: Rapid development cycles

• Scenario: Development teams practicing XP need shorter, more frequent release cycles. Iterations help structure this fast-paced workflow while maintaining visibility.
• Task: Implement code changes in 1-week cycles with frequent releases.
• How iterations are used: Configure weekly iteration cadence, use iteration boards to track development status, monitor velocity through iteration analytics.
• Benefit: Enables faster feedback loops while keeping work organized and measurable.

Related issues

• #220942 (comment 2280808454)
• technical-writing#1144

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
• Follow the:
  • Documentation process.
  • Documentation guidelines.
  • Style Guide.
• If you're adding a new page, add the product availability details under the H1 topic title.
• 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 you aren't sure which tech writer to ask, use roulette or ask in the #docs Slack channel.

• If the content requires it, ensure the information is reviewed by a subject matter expert.
• Technical writer review items:
  • Ensure docs metadata is present and up-to-date.
  • Ensure the appropriate labels are added to this MR.
  • 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.