update utility docs

[clmould]

Description

The utilities section of the documentation was out of date. It has been updated to remove utilities which no longer exist, and to update the instructions on remaining utilities if needed. A few other minor changes have been made such as creating a new sankey plot which now renders on github, and removing the plot sankey full option which did not work.

Checklist

I confirm that I have completed the following checks:

• I have justified any large differences in the regression tests caused by this pull request in the comments.
• I have added new tests where appropriate for the changes I have made.
• If I have had to change any existing unit or integration tests, I have justified this change in the pull request comments.
• If I have made documentation changes, I have checked they render correctly.
• I have added documentation for my change, if appropriate.