Update assessment notes to mention single-source requirement

[chalin]

Update the assessment notes to require that the source files for all website pages should reside in a single repo (we've been referring to this as the single-source requirement), ideally in the website repo itself (this is the simplest solution), or brought in via git submodules.

Per discussion with @celestehorgan on 2021-10-23

[nate-double-u]

+1, I like the idea of including this here.