Blog about DataFusion correlated subquery support

### Is your feature request related to a problem or challenge?

This will serve as the final report / writeup for our Google Summer of Code 2025 project
- https://github.com/apache/datafusion/issues/16059

This will serve both to draw more attention to DataFusion as well as demonstrate how well the project went

### Describe the solution you'd like

The idea is to write a blog post on https://datafusion.apache.org/blog/ that summarizes:
1. The high level problem of running decorrelated subqueries
2. The results of the Summer project
3. A sketch of how it was completed (both technically and with others)

### Describe alternatives you've considered

Code is in https://github.com/apache/datafusion-site

I suggest we start working on the post as soon as possible as it takes quite a while to write such content

### Additional context

cc @irenjj

Start by reviewing issue #16059 for the correlated-subquery project results and the datafusion-site repository for the blog structure and contribution process. Draft a post covering the high-level decorrelation problem, Summer of Code results, and the technical and collaborative approach; it is done when that post is added to the DataFusion blog.

Written by the indexing model from the issue text.