Clean up Cairo and Imagemagik dependencies

Doing a local handbook build on Mac is currently difficult due to the need to set up imagemagik and Cairo. We also have two Cairo libraries we are using.

This issue has the following tasks:

• Make the imagemagik dependency optional - only fail if it is needed for pdf generation.
• Consolidate the use of Cairo/PyCairo so we only use one
• Document how to install the dependencies on Mac, including any edge cases.