Update documentation to cover new napari-tomotwin features

[thorstenwagner]

With the upcoming napari-tomotwin, the clustering workflow becomes more streamlined and powerful. This needs to be reflected in the documentation before I can release it.

Some points that needs to be reflected in the documentation:

• [] How to recalculate a umap
• [] How and when to use "preview targets"
• [] Strategy 1 is now basically obsolete for the clustering workflow