Add examples from "Causal Inference and Discovery in Python" to the tests to ensure backwards compatibility

[AlxndrMlk]

Is your feature request related to a problem? Please describe.
As discussed with @amit-sharma and @emrekiciman, we want to add code examples from the book "Causal Inference and Discovery in Python" to the DoWhy's test suite to ensure backwards compatibility between Dowhy 0.8 and future releases.

Additional context
Backwards compatibility will help existing users smoothly migrate to new releases without being forced to adapt to new APIs.

[AlxndrMlk]

@amit-sharma @emrekiciman - I'll schedule this right after we close #772

[rahulbshrestha]

Hey @AlxndrMlk, can I help out in this PR? :)

[amit-sharma]

@rahulbshrestha will be great if you can contribute. This will be a very valuable addition to the community, especially readers of @AlxndrMlk's book

[AlxndrMlk]

Hey @AlxndrMlk, can I help out in this PR? :)

@rahulbshrestha -- thank you for proposing, that would be great!

Please let me know if there's anything you'd like to know before we start.

@amit-sharma - 💯

[rahulbshrestha]

@AlxndrMlk Yes, thanks! I'm new to this repo so any help would be appreicated :) Which tests do would you like to be added and in which test file in dowhy should I put it? Are there any existing examples already on dowhy I can look at?

[amit-sharma]

@rahulbshrestha you can start by looking at the tests in the tests/ folder as examples.
Then, I suggest creating a new folder tests/causal-inference-discovery-book and then add examples from the book as test cases. You can add 1-2 examples and create a PR and we can give you feedback.

Once the basic code structure is set up, it will be faster to add the other examples. If you have more questions, feel free to message me on discord (you will find the discord link on readme)