Clean up documentation

[JanCBrammer]

Archive "dead" and legacy docs

Distinguish "living" (i.e., those docs that will be edited / extended during development) from "archive-able" docs, and move the "archive-able" docs out of the way (e.g., sub-directory, external archive) to de-clutter the repo.

File formats

Where possible, unify documentation file format. Preferably Markdown, or another editable plain-text format. Currently, a majority of the docs are in non-plain-text formats (.docx, pdf). The latter formats cannot be edited (conveniently), and they won't be displayed on GitHub. That is, those formats add unnecessary friction to reading and editing docs.