Update group-by example file.

[shb8086]

This pull request updates the examples/group-by.md documentation to provide clearer guidance and a step-by-step example for grouping check results by category in Soda Core. The changes aim to make the example more instructive and accessible for users looking to identify patterns in data quality issues.

Documentation improvements:

• Expanded the introduction to explain the usefulness of grouping failed check results by category, emphasizing how it helps identify data quality patterns.
• Reorganized the example into a step-by-step format, clarifying each stage of building and testing the SQL query before implementation in Soda Core.
• Improved the formatting of code blocks and images for better readability and consistency throughout the example.

[CLAassistant]

All committers have signed the CLA.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud