Add reusable Footer component matching whitehouse.gov layout with custom

  branding

  - Created Components.Footer in components.js as universal footer component
    - Configurable basePath option for pages at different directory levels
    - Supports custom site name, address, and brand link configuration
    - Safari-compatible using Object.assign() instead of spread operator

  - Restructured footer to match whitehouse.gov design:
    - 2-column main grid with left nav and center section
    - Center section uses 3 sub-columns: nav | logo/newsletter |
  address/social
    - Bottom bar with flush edges and vertical separator before "Back to
  top"
    - Proper whitespace and alignment matching government site pattern

  - Updated newsletter section layout:
    - Side-by-side flexbox layout with text left, form right
    - Maintains custom pink gradient background and white text
    - Preserves original input/button styling with rounded corners

  - Added footer documentation at docs/FOOTER-COMPONENT.md
    - Usage examples for root and subdirectory pages
    - Configuration options reference
    - Path handling guide

Author: radicalkjax