feat(website): minor various improvements to website and docs

[dionysuzx]

• added an announcement for testnet end date
• fixed weird footer
• remove dark mode toggle from homepage (weirdly placed, if we do this we can do it properly later, with a toggle in the menu bar)
• centered the tagline / animation for homepage on large screens (too much width, hard to read and stay focused vertically)
• refactored documentation to fit within the framework we'll use
• renamed testnet links in the header so it will work better for next testnet