Small doc fixes

[B-Deforce]

Checklist

• [N/A] I've formatted the new code by running hatch run dev:format before committing.
• [N/A] I've added tests for new code.
• [N/A] I've added docstrings for the new code.

Description

The example notebooks contained some doc issues, which have been resolved in this PR.

• Data Synthesis
  • Part of some paragraphs was wrongfully assigned as metadata/title, as a result it didn't show up in the notebook (e.g., this paragraph)
  • Contained broken link in last paragraph
• Placebo Testing
  • Contained redundant cells
• AZCausal Integration
  • Part of some paragraphs was wrongfully assigned as metadata/title, as a result it didn't show up in the notebook (e.g., this paragraph)

Issue Number: N/A