Add structured overview of new 1.0 features as a Migration Guide

### Problem

There is no structured overview of new features introduced in DataFrame 1.0.

Users upgrading or evaluating the library cannot easily understand:
- what has changed
- what is new
- what is important to learn first

### Expected

Provide a clear overview of new features introduced in 1.0.

### Acceptance criteria

- A dedicated overview of new features is created
- It is available in:
- website documentation (topic/page)
- at least one notebook with runnable examples
- Features are grouped (e.g. compiler plugin, IO, DSL changes, etc.)
- Each feature includes:
- short description
- minimal usage example
- Content is linked from the migration guide

### Motivation

- 1.0 is a major release
- Users need a quick way to understand what changed
- Improves onboarding and adoption of new APIs

Start by reading the existing migration guide and locating the website documentation topic and notebook conventions in the repository. Inventory the DataFrame 1.0 features, group them by area, add short descriptions with minimal runnable examples, and link the overview from the migration guide.

Written by the indexing model from the issue text.