Update docs/transform.ipynb based on new transformer framework. (#5154)

Fixes #4960


This notebook provides a high level overview of all the newly added features to Cirq's transformer infrastructure. We can discuss whether we need more details docs on each of the specific topics -- I think a high level overview with links to reference pages of individual methods (i.e. the current PR) should be enough to close #4960, but I'm curious to hear what others think.