Migrating developers' tutorial to public pages: part 1 (prerequisites)

This MR is the first of several MRs to migrate the developers' tutorial from Twiki to the public MkDocs pages, where they should be. This is the pre-requisites part; I'm doing this one first to check we're happy with the overall layout; once this is merged I'll do the rest over the coming days. No changes to the content w.r.t the Twiki.