Improve changelog format, internal developer documentation

[brookslogan]

Taken from discussion on #222.

• Consider regrouping entries: comment 1, comment 2. Or consider tidyverse news style.
• "Here and elsewhere: try to limit to a small number of verbs. Maybe a recommended verb set, although some of these are trying to accomplish what the categories already do, but we might move from categories -> "importance", and then these would still be useful even though they might look redundant now."
• Look at decision/comments in Add changelog (NEWS.md), associated pkgdown styling #222 on adding a dev branch and what the default branch should be, and implement them.
• Add an (epiproess) developer doc describing the branching and version release procedure.
• Turn the retrospective version branches into release version tags.

[dshemetov]

FYI: https://keepachangelog.com/en/1.1.0/ (h/t @melange396)