Notebook explaining what BO is and how it works

[joelberkeley-secondmind]

As a user new to Bayesian optimization, I would like a notebook that explains what Bayesian optimization is and how it works, so I can start using it to solve my optimization problems with little experience.

As a maintainer, I want the above so that people with little experience of BO use the project as well as those with lots of experience, thus promoting a larger community.

[uri-granta]

The expected improvement tutorial essentially does this (which will be made more clear following #769). It could definitely use improvement, but this is covered by #472.