Add Judge Builder blog post

[danielseong1]

Summary

• Adds a blog post announcing the Judge Builder feature in MLflow 3.9
• Judge Builder is a visual interface for creating custom LLM-as-a-Judge evaluators directly in the MLflow UI

The blog post includes:

• Introduction to the feature and its value proposition
• Step-by-step guide to using Judge Builder
• Configuration options table
• Link to detailed documentation

Note: This PR was originally filed in the main mlflow repo (mlflow/mlflow#20353) and moved here per reviewer feedback.

🤖 Generated with Claude Code