Modernize homepage design

[SharbelMarshi]

This PR refreshes the GNU Octave homepage while preserving the existing static Jekyll/Foundation structure. Changes:

• Reworks the homepage into a clearer modern landing page.
• Adds a stronger hero section with Download, Documentation, and Get Involved calls to action.
• Adds high-quality SVG plots
• Adds feature sections for numerical computing, plotting, packages, MATLAB-compatible scripting, and community/development.
• Adds responsive CSS for mobile layout.
• Updates outdated links that were not opening correctly

Technical notes:

• No build dependencies were changed.
• No JavaScript framework was added.
• No analytics, tracking, external fonts, or new CDN dependencies were added.

Tested locally with: bundle 2.4.22 exec jekyll serve

[siko1056]

Thank you very much for your interest in updating the GNU Octave homepage @SharbelMarshi . As this website belongs to the Octave community, I think it is best to advertise your ideas about the new design here first: https://octave.discourse.group/c/community/9