Update xgboost documentation following refactor

[jmogarrio]

49 and #56 aimed to clarify the structure of the code, but now it might be helpful to do the following:

• [x] update the README if needed;
• [x] make sure the notebook text still makes sense;
• [x] make sure the library packages are properly documented.

[benwulfe]

JMO, this looks like a task that you want to do as a follow on to the refactor. Assigning to you for now, but before taking action, let's review if we intend to change our primary colab based on Ruben's work.

[jmogarrio]

JMO, this looks like a task that you want to do as a follow on to the refactor. Assigning to you for now, but before taking action, let's review if we intend to change our primary colab based on Ruben's work.

Sorry, missed this comment before taking action, this task is effectively done. Will follow up to understand how Ruben's work affect's the colab.