docs: overhaul README with hero, badges, vision, and community CTAs

[markovejnovic]

Summary

• Rewrites README from dry reference doc into a compelling landing page (modeled after starship/uv/bun/just)
• Adds centered hero with badges (CI, crates.io, Discord, Slack, license), warning banner about alpha status, elevator pitch with inline demo, feature highlights, vision section, and community CTAs
• Moves reference material (DSL surface, flags, cloud commands, plugin authoring) behind collapsible <details> sections
• Primary install method is now curl -fsSL https://get.harmont.dev/install.sh | sh

Test plan

• Badges render correctly on GitHub (CI, Discord widget enabled, Slack)
• All links resolve (harmont.dev, discord, slack invite, crates.io, pypi, examples/)
• [!WARNING] admonition renders with GitHub styling
• <details> sections expand/collapse
• Centered HTML renders on GitHub