Finalize and document CIL bridge

[SimonRit]

@epapoutsellis has started an illustration of the use of RTK in CIL with TomographicImaging/CIL-User-Showcase#35. We should help finalize and link RTK to this.

[epapoutsellis]

Hi @SimonRit, I will create a PR in the next days but I will definitely need your help for the RTK side