Write 'Hedera Services' overview (learn/core-concepts/services/index)

**Page:** `learn/core-concepts/services/index.mdx`

**Context:** Siblings `tokens.mdx` and `smart-contracts.mdx` are written; the description promises "tokens, consensus, files, and smart contracts" but only 2 service pages exist.

**Scope:** Conceptual overview of the core services.

⚠️ **Gap to resolve:** consensus and files service pages don't exist under `services/` — decide whether to add them or link to `native/consensus` and `native/files`.

### 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)