High level documentation of the graph model

[jgsogo]

A high-level view of the graph model: build requires, privates, overrides, conflicts,... tools, testing libraries. Maybe an example project... Talk about all these things in a comprehensive section could help with issues like: #1439 and conan-io/conan#5684 (comment) or some of the ones listed here https://github.com/conan-io/conan/projects/8