Updates to the base theme

Why is this change being made?

All layouts with the exception of the one supporting the homepage have been moved to the theme so they can be shared across handbooks. The same applies to the contribuitor docs.

The handbook has been tweaked to remove references to legacy handbook to current public handbook to avoid any confusion and the background on the top cover has been altered slightly.

Author Checklist

• Confirmed that this update should be in the internal handbook rather than public handbook, per the SAFE framework
• 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.
• 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
• 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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