Native Quickstart — landing/router (native/quickstart/index)

**Page:** `native/quickstart/index.mdx`

**Context:** Siblings `go`, `java`, `javascript` quickstarts are written.

⚠️ **Likely a thin language-picker router, not a full content page.** Scope: short intro + `` to the three language quickstarts.

### Acceptance criteria
- [ ] Replace the placeholder template with a real landing page
- [ ] Remove `hidden: true` from the frontmatter
- [ ] **No "Browse by section" block** — the side nav covers that; open straight into the deep-dive sections
- [ ] Route into the written sibling pages via ``/``
- [ ] Follow the style guide (markdown tables only; no AI-tell phrasing)