Sherpa_i: Update documentation for Sherpa 3.x
Several parts of the Sherpa_i README's and example files were out of date, in particular (but not only) due to the switch to Sherpa v3.x and due to links pointing to the old 21.6 branch.
Several parts of the Sherpa_i README's and example files were out of date, in particular (but not only) due to the switch to Sherpa v3.x and due to links pointing to the old 21.6 branch.