Use Zensical card grid on docs homepage

[tobiasraabe]

Summary

• replace the custom homepage tile HTML with Zensical's native card grid syntax
• preserve the original card copy and icons while keeping the grid at 2x2 on larger viewports
• simplify the custom CSS to only the homepage-specific overrides still needed

Verification

• rendered the docs homepage locally with just docs-serve
• captured a local screenshot to verify the homepage renders as four separate cards again