Move public handbook homepage styles to scss theme files

Why is this change being made?

Recent updates to the public handbook homepage in MR!16784 had a style block for all styles. In order to keep things consistent and clean, all CSS is being moved to the _handbook.scss file.

Tested locally by moving all styles in the handbook repo from /content/_index.html to the docsy theme repo /assets/scss/_handbook.scss. Verified changes in both light and dark theme mode.

• Move public handbook homepage styles to scss theme files

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)
  • 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
  • Handbook: For changes that affect handbook users, but does not need company-wide attention, post in the #handbook 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

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