Add some high level steps for a sunset roadmap

Why is this change being made?

From https://gitlab.com/gitlab-com/gl-security/appsec/tokinator/-/issues/76

📓 📓 No pressure, but if you had any lessons learned from this process and/or think there's anything helpful to capture in the handbook or a template, please do consider making an MR! I realize it might just be "do the thing!" and not really worthy of a step-by-step description or template, though.

This MR adds some guidance around sunset roadmaps and links to recent (at time of writing) examples. It's intentionally not prescriptive because a sunset roadmap really is kinda a "do the thing!" type effort. But documenting the top things that came to mind might help others in the future. 🤷

It also fixes a broken link

Author and Reviewer Checklist

Please verify the check list and ensure to tick them off before the MR is merged.

• Provided a concise title for this Merge Request (MR)
• Added a description to this MR explaining the reasons for the proposed change, per say why, not just what
  • Copy/paste the Slack conversation to document it for later, or upload screenshots. Verify that no confidential data is added, and the content is SAFE
• Assign reviewers for this MR to the correct Directly Responsible Individual/s (DRI)
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the Maintained by section on the page being edited
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies
  • The when to get approval handbook section explains the workflow in more detail
• For transparency, share this MR with the audience that will be impacted.
  • Team: For changes that affect your direct team, share in your group Slack channel
  • Department: If the update affects your department, share the MR in your department Slack channel
  • Division: If the update affects your division, share the MR in your division Slack channel
  • Company: If the update affects all (or the majority of) GitLab team members, post an update in #whats-happening-at-gitlab linking to this MR
    • For high-priority company-wide announcements work with the internal communications team to post the update in #company-fyi and align on a plan to circulate in additional channels like the "While You Were Iterating" Newsletter

Commits

• Add some high level steps for a sunset roadmap

---