move clusterctl to main toolbox

This is needed to be able to use clusterctl in debug-on-exit (see sylva-projects/sylva-core!2555 (merged)).

(the README.md is updated in this MR to reflect the move of clusterctl to the base image; I also changed the way the numbering is done so that re-numbering won't be necessary in the future, leveraging the ability of markdown to automatically number numbered-list items).