Refactor TW handbook pages and remove duplicate content

What does this MR do and why?

• Integrates Documentation and Technical Writing Fundamentals completely into the Technical Writing handbook page:
  • Adds a new Documentation section to Technical Writing and links to the following docs sections:
    • Documentation is the single source of truth (SSoT)
    • Docs-first methodology
  • Expands the Public training for GitLab Technical Writing section and links to both the GitLab Technical Writing Fundamentals course in Level Up and the feedback issue
• Links to the new docs-gitlab-com project instead of the deprecated gitlab-docs
• Deletes the now-redundant Documentation and Technical Writing Fundamentals pages and fixes all broken links

Related issue

• gitlab-org/technical-writing/team-tasks#1281 (closed)

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

• Refactor TW handbook pages and remove duplicate content

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