Update documentation on registration

[DanicaSTFC]

After merging PR #206 the documentation has not be updated. This is now outdated to the manual registration method.

Code location https://github.com/TomographicImaging/iDVC/blob/4fc64e6d3bcc6ce53e5b71723225f1c137c82f79/docs/source/dvc_steps.rst?plain=1#L7-L10

and

https://github.com/TomographicImaging/iDVC/blob/4fc64e6d3bcc6ce53e5b71723225f1c137c82f79/docs/source/dvc_steps.rst?plain=1#L20