Merge cinnabar-absolute and cinnabar RBFE plotting docs

[jthorton]

https://github.com/OpenFreeEnergy/ExampleNotebooks/blob/main/cookbook/cinnabar_absolute.ipynb is currently not used in the openfe docs, we should merge it into https://github.com/OpenFreeEnergy/ExampleNotebooks/blob/main/plotting_rbfes_with_cinnabar/PlottingFreeEnergiesUsingCinnabar.ipynb to make a general plotting with cinnabar notebook with sections for RBFE and ABFE result plotting starting from outputs from the openfe CLI.