Fix numerous docs and build issues

[jni]

• The complete analysis page in the docs appears to have no output. This was because the sample data was in the wrong location after the docs got rearranged into folders.
• Fix 3D Labels view is broken in docs #179 3D labels view not working, which was caused by pinning napari to an old branch; this was fixed by moving to napari<0.4.18.
• Fix numerous broken links caused by the folder rearrangement
• Fix deprecation warning messages from imageio
• Fix incorrect sphinx configuration dating from switch from material to pydata
• Fix GitHub actions deprecations by using aganders3/headless-gui action
• Remove a bunch of unused files
• Update GitHub actions versions for many action dependencies
• Use nicer (to my eyes) older pydata-sphinx-theme==0.8.1
• Remove outdated requirements.txt files