Try thinking about this in a different way:
Who is going to be reading this document? What do they expect to see in it? Ask them. And then ask for feedback when your first draft is completed.
Tech design documents vary widely in structure and form, between organizations and even within them. Take your best guess, spend a couple hours writing up a very rough draft, then ask your colleagues to tell you what's missing or what's redundant, or what needs more or less detail. Then iterate and make it better.
It sounds like your organization doesn't have a lot of experience with formalized procedures for software design. You have a golden opportunity here to establish yourself as the expert.