feat: Sharp style, Tailwind+shadcn, dark navbar, nav restructure, Go/monorepo guidelines

Styling:
- Tailwind CSS v4 (preflight:false) integrated via PostCSS plugin
- shadcn/ui New York style with --radius:0 (sharp corners everywhere)
- Zero border-radius on ALL elements: buttons, code blocks, cards, admonitions, badges
- Navbar FORCE dark (#0a0a0a) in both light/dark modes via !important
- ThemeSynchronizer: bridges Docusaurus data-theme → shadcn .dark class
- Active sidebar item gets left border accent instead of rounded bg

React UI components (src/components/ui/index.tsx):
- Badge (6 variants: wip, stable, planned, success, warning, outline)
- Card + CardHeader + CardTitle + CardContent
- Callout (note/info/success/warning/danger) - sharp, left-border style
- PropertiesTable - monospace typed API reference tables
- StatusIndicator - operational/degraded/outage/maintenance

Navigation restructure:
- Introduction → Platform → Products → Engineering → Runbooks
- Old 'applications/' removed, moved to products/stock-market-pro/
- New Platform section: infrastructure overview + CI/CD pipeline docs
- New Engineering section: monorepo structure, Go stack, conventions
- New Runbooks section: incident severity, common procedures

Implementation guidelines (engineering/guidelines.md):
- Monorepo layout with go.work workspace
- Go as primary language (why + standard library table)
- Python as sidecar for data/ML workloads
- Code conventions: gofmt, error wrapping, logging, testing
- Git conventions: branch naming, conventional commits
- Makefile targets per app
- How to add a new application (step-by-step)

components.json

@@ -0,0 +1 @@
+{"$schema":"https://ui.shadcn.com/schema.json","style":"new-york","rsc":false,"tsx":true,"tailwind":{"config":"tailwind.config.js","css":"src/css/custom.css","baseColor":"zinc","cssVariables":true,"prefix":""},"aliases":{"components":"@site/src/components","utils":"@site/src/lib/utils"},"iconLibrary":"none"}