Revamp Mintlayer docs with better UI and step marking

[Kali-Decoder]

Summary

• Migrated Mintlayer docs from Docusaurus to Astro Starlight for a cleaner, darker docs UI
• Kept existing Mintlayer content (whitepaper, Mojito, SDK, MCP, bridge, guides, Trezor) with updated frontmatter and sidebar structure
• Added a branded home experience (hero + track cards) and clearer navigation paths for builders and guides

Test plan

• Run npm install && npm run dev and open the home page
• Confirm sidebar sections: Start Here, Build, Guides, Whitepaper
• Spot-check pages: Intro, Mojito Inject, JS SDK, MCP, Issue Token, Bridge, Trezor
• Verify whitepaper cross-links navigate correctly
• Confirm callouts/admonitions render correctly (notes, tips, cautions)
• Run npm run build and confirm it completes successfully

cc : @erubboli @anyxem Hey want to know your feedback and thoughts !