Drop numeric XX- from markdown file names

[effigies]

Your idea

We've already done this with appendices in #1256. The numbers required placing appendices in the order they were added to the spec, not following any kind of logical or aesthetic order.

I think the modality-specific files in particular are suffering from this ordering: There are some groupings by similarity (volumetric MRI + PET, detector array *EG + NIRS) that would make sense to reflect in the section ordering. Additionally, when we have multiple BEPs running, they can clash on section numbering, which is simple to resolve but unclear why we should need to.