docs: add eggl-cli documentation site

[roberteggl]

Summary

• add an Rspress documentation site for eggl-cli
• document installation, configuration, commands, workflows, troubleshooting, development, and releases
• add a terminal-style homepage hero and custom visual theme
• deploy the docs to cli.eggl.dev through GitHub Pages
• update README branding, attribution, and Homebrew homepage metadata

Verification

• make fmt && make check
• go vet ./...
• make build
• npm ci && npm run build

[codecov]

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