Add legal footer to specs docs

[krofax]

Description

Add legal footer to specs docs

Overview

This PR adds a standardized legal footer to the spec site, addressing the legal team's requirement to include essential privacy policy information and legal disclaimers across all OP documentation sites.

Changes

• Added footer component with required legal links and copyright notice
• Created responsive footer layout that adapts to different screen sizes
• Integrated with existing mdBook theme system using CSS variables for consistent styling
• Implemented using vanilla JavaScript to avoid external dependencies

Implementation details

• Created footer.css and footer.js in the theme directory
• Footer includes:
  • Community Agreement link
  • Terms of Service link
  • Privacy Policy link
  • Copyright notice: "© 2025 Optimism Foundation. All rights reserved."
• Maintains responsive design down to mobile screen sizes

Screenshots

Related Issues

Closes - https://github.com/ethereum-optimism/devrel/issues/506

Tests

Additional context

Metadata

[clabby]

This PR proposes to violate the license of this repository, I think (ianal.) We waived all rights to copyright the contents of this repository with the CC0 license, it exists in the public domain.

https://creativecommons.org/publicdomain/zero/1.0/deed.en

The person who associated a work with this deed has dedicated the work to the public domain by waiving all of his or her rights to the work worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
You can copy, modify, distribute and perform the work, even for commercial purposes, all without asking permission.