styles: completed upgrade homepage and others updated

[Tony133]

Proposals:

• Replaced the generic logo with dedicated black/white variants for light/dark theme, removed the unused Docusaurus placeholder asset
• Darkened the Core Features cards background in dark mode
• Fixed hero background tone in light mode to match the design system's #f5f5f5 (was landing on #dfdfdf due to the invert filter)
• Fixed the Sponsors box background, which was hardcoded to a different gray than the rest of the site
• Extended the "hero's hexagon" pattern to the other dark sections for visual consistency
• Added a count-up animation for the hero stats (GitHub stars / monthly downloads)

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Screenshots ( Preview )

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cc @fastify/website