There are docs resources like
https://diataxis.fr/ that categorise documents based on format and intended audience.
They don't say where you should start, or what order you should write docs.
I'm currently thinking README > reference > tutorial > how-tos. Agree/disagree?
I often use Quip for writing documents and I've started using Google Slides for presentations. They have fewer features than competing tools, but I like them.
They have what I need, without being overwhelming. Maybe it takes a paradigm shift (e.g. desktop->web) to drop features.
A critique of XML usage, and annotating documents rather than exchanging data:
https://www.devever.net/~hl/xml
It's rare to see discussions of good XML usage! I've seen marked-up word definitions (a multilingual dictionary) that were a nice fit for XML, but it's rare.