Add notebook examples to docs

[jeongyoonlee]

Proposed changes

This PR adds example Jupyter notebooks to the documentation website at https://causalml.readthedocs.io/.

• Rendering example notebooks on the Sphinx document website
• Updating example notebook titles to be informative
• Organizing example notebooks

Types of changes

What types of changes does your code introduce to CausalML?
Put an x in the boxes that apply

• 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

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your code.

• 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

This PR moves the examples folder from the project root to docs/ because the nbsphinx extension cannot render notebook files outside the docs/ folder. There is another extension, nbsphinx-link, that enables rendering notebooks outside docs, but it requires creating one .nblink for each notebook file to be rendered. Also the nbsphinx-link project has been inactive in the past 4 years.

In short, I found moving the entire examples folder is the simplest solution.

[ras44]

LGTM 👍

[ras44]

LGTM

[ras44]

LGTM- I like the quickstart idea