Clean up low-res images in Composer documentation

[abbycross]

Followup project to generate higher-res images, and fix the incorrect image spotted in #5132 (it is being temporarily removed there; see the original here. Third circle should say "Maximally mixed" rather than Maximally entangled")

[mackblank]

Here is the diagram w/ the label fixed so that it can go back into the page - we are still working on updating the other low res diagrams