Fix the readthedocs build error

[jeongyoonlee]

Proposed changes

This PR fixes the readthedocs build error. Currently, the doc build is failing due to a Unicode error. I added the latex_engine config to docs/conf.py as suggested here.

I also fixed inconsistent section levels in some of the example notebooks.

Types of changes

What types of changes does your code introduce to CausalML?
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• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices apply)

Checklist

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• I have read the CONTRIBUTING doc
• I have signed the CLA
• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)
• Any dependent changes have been merged and published in downstream modules

Further comments

[ras44]

LGTM- thanks!