Native SDKs Overview — landing page (native/overview/index)

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

**Scope:** Intro to the JS/Java/Go/Rust SDKs; route into `why-native-sdk`, `sdk-comparison`, and the Quickstart.

### 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)