Add renders of the sample clip assembly, and of the whole microscope

The sample clips page currently has no picture. This adds two renders of the microscope with sample clips to the relevant assembly page.

I have also added some renders of the "completed" microscope, including the electronics, though not including screws to mount the electronics boards. This could be used to add renders to the electronics step, though currently Julian's video is probably better.

I have added a "you are finished" page at the end of the instructions, with pictures of the completed microscope, and links to the software instructions. More needs to be done about usage instructions and software instructions, but this is a decent start, I think.

I have also explained in a README how to preview the instructions directly from a merge request, in case others want to contribute instructions-related MRs.