Improve tracer_model_split documentation in pipelining tutorial

[patrocinio]

• Added clear section headers for both splitting options
• Option 1: Manual Model Splitting
• Option 2: Tracer-based Model Splitting
• Fixed typo: 'before the before' -> 'before the'
• Added explanation of split_spec dictionary parameters
• Clarified that split_spec specifies module path and split point type
• Made the tracer_model_split function definition more prominent

The tracer_model_split code block was present but users were missing it because it wasn't clearly labeled as a separate option.

Fixes issue #3530

Description

Checklist

• [ ] The issue that is being fixed is referred in the description (see above "Fixes #ISSUE_NUMBER")
• [ ] Only one issue is addressed in this pull request
• [ ] Labels from the issue that this PR is fixing are added to this pull request
• [ ] No unnecessary issues are included into this pull request.