Add more information about AssetBundle formats to Documentation/unity-content-format.md

[SkowronskiAndrew]

We have been adding a lot more helpful general information about AssetBundles into the AssetBundle section of the Manual, e.g.
AssetBundle File Format

However when getting to the point of using UnityDataTools to analyze AssetBundle builds it can be helpful to have a deeper understanding. This could be added to unity-content-format.md (possibly branching out to additional .md files). The format is very stable and well established so at this point it won't be hard to maintain so long as any content we publish is accurate.

This task is to expose more information, diagrams etc of some of the key datastructures and concepts (based on internal Unity documentation)