Add CI/CD project template & workshops to DE handbook, update OSS&projects

Why is this change being made?

This MR documents a new template project and updates the related areas around projects and OSS contributions to the current state, making it more precise as a SSoT.

• Go Tanuki as CI/CD workshop template - https://gitlab.com/gitlab-de/workshops/gitlab-cicd-go-tanuki-workshop in the workshops group at https://gitlab.com/gitlab-de/workshops
• Details on past workshops and meetups - from gitlab-com/marketing/corporate_marketing&49 (closed)
• OSS contributions and projects overhaul

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
• Assign this change to the correct 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 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 relate to any part of the project other than updates to content and/or data files please make sure to ping @gl-static-site-editor in a comment for a review and merge. For example changes to .gitlab-ci.yml, JavaScript/CSS/Ruby code or the layout files. (this requirement has been removed pending identification of a new DRI for the handbook)