Reorganise and improve federated merging docs

[t92549]

#319 added docs around schema merging. This is different to the FederatedOperation docs. This should be reorganised and in one structure. Additionally, extra docs should be added on the other merge functions (for merging traits or elements)

[GCHQDeveloper314]

Could be considered alongside #187 as part of a refactoring of this area of the docs.