docs: add VitePress documentation site

- 22 content pages across Guide, Concepts, and Reference sections
- Custom indigo/cyan theme with Lucide icons and Mermaid diagrams
- GitHub Actions workflow for GitHub Pages deployment
- Live preview: https://mempalace-docs.netlify.app/

website/.vitepress/theme/index.ts

@@ -0,0 +1,6 @@
+import DefaultTheme from 'vitepress/theme'
+import './style.css'
+
+export default {
+  extends: DefaultTheme,
+}