Create system blueprint for existing Saas website

I currently have a large SaaS website and I'd like to hire a Solution/Product architect that has extensive experience with creating system blue prints. I'd like the blue prints to map out user workflow, how various functions work together with each other, functionality of each page and feature, api's, database integration, etc.... I'm not sure what the standard is for these types of documents, so I will rely on the experience and recommendations of the person handling this to tell me what developers are normally looking for in these documents. I have several developers working in the system and my goal is to provide them with a "living and breathing" document that we can update if we make any system changes, but new developers can refer to it to understand how various features work, making it easier to debug issues and gain a really good understanding of how the user and development portions or the system work.