Add alt text for images in ERM chapter

For each image in the ERM chapter, I:

• gave it an alias in the chapter instead of calling the image file directly;
• created the alias in images.rst;
• added alt text in images.rst.

Every. Single. Image. All 85 of them 😭

Also, I tried to rebase half-way through; hope I didn't make a mess of things... 😬