Add new dashboards project to Developer Evangelism projects handbook

Why is this change being made?

This MR adds the new dashboards project from https://gitlab.com/gitlab-de/dashboards into the Developer Evangelism projects handbook.

Applied structural changes to move all projects into the "OSS Projects" chapter, and make them more prominent.

First documentation step for https://gitlab.com/gitlab-com/marketing/corporate_marketing/corporate-marketing/-/issues/5977+

Author Checklist

• Provided a concise title for the 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.
• Assign reviewers for this change to the correct DRI(s)
  • 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 in 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.
• If the changes affect team members, or warrant an announcement in another way, please consider posting an update in #whats-happening-at-gitlab linking to this MR.
  • If this is a change that directly impacts the majority of global team members, it should be a candidate for #company-fyi. Please work with internal communications and check the handbook for examples.

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