phased-updates: first pass of technical updates

[Hyask]

That documentation is written like a live document, more or less playing the role of a bug tracker with open questions and future work. This doesn't fit a documentation page, so I've removed most of those entries, and corrected what I could so that at least the technical information in that page is correct. This still needs an overhaul to better fit modern documentation standards.

cc: @s-makin

[s-makin]

Thanks for reviewing this @Hyask - I agree on dropping the block you suggested, if you want to do that as part of this PR I'll hold off on merging, otherwise I can do it as a followup (lmk which you prefer)

[rkratky]

Thanks, @Hyask. A few suggestions and a question.

[Hyask]

Thanks both for the review. I've addressed your comments and suggestion, added a link to the phased-updater script, and dropped a few blocks.
I still think this would need some proper structure from a technical author, as right now, this still feel like half a spec, half a bug tracker, and another half of end-user documentation. That many halves can't be good for sure 😀