Update index.md to include atomic elements section

[rami-elementor]

Added a note about atomic elements to the index.

✨ PR Description

1. Problem & Context

Replaced unclear section heading "Atomic Structure" with descriptive introductory text to improve documentation clarity and navigation.

2. What Changed (Where

src/data-structure/index.md: Changed heading from "Atomic Structure" to "For information about atomic elements:" with added blank line.

3. How It Works

Removed single-line heading and replaced with prefatory sentence followed by blank line, then list items remain unchanged structurally.

4. Risks

None — purely documentation formatting. No impact on build, links, or functionality.

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