Write 'ATS Quickstart' (solutions/tokenization/ats/quickstart)

**Page:** `solutions/tokenization/ats/quickstart.mdx`

**Context:** Siblings `index`, `web-ui`, `faq` are written.

**Scope:** Get started with Asset Tokenization Studio — install/deploy, create a first security token. Pairs with the existing ATS pages.

### Acceptance criteria
- [ ] Replace the placeholder template with real content
- [ ] Remove `hidden: true` from the frontmatter
- [ ] Follow the style guide: **markdown tables only** (no HTML ``), and avoid AI-tell phrasing (no em-dashes, "delve", "leverage", "not just X but Y")
- [ ] Add working cross-links to related pages
- [ ] Confirm any code samples run on testnet