Update the handbook homepage with new TOC

Why is this change being made?

This is a big update. This updates the homepage to add search at the top, remove the old migration warning and adds a revised ToC based on a data file. In addition we've added a bootstrap/javascript notification system for showing the new migration notice which will show on all pages once a week. The full feature list is:

• Implemented the Handbook Changelog page.
• Implemented Notifications
• Implementation Notification preferences
• Implemented Notification creation tool
• Implemented Data based Table of Contents
• Reword the homepage
• updated Docsy to version 7.1 for various bug fixes.

Screenshots

New homepage with notification

New changeling page

New handbook preferences page

Author Checklist

• 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 post 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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