Adds a second Marp source root. Decks under slides/ build to an output path
mirroring their repo path, so slides/TAPPaaS/HOW-TO/NewModule/index.md is
served at slides.makerfloss.eu/TAPPaaS/HOW-TO/NewModule/. Decks in
docs/presentations/ keep their flat URLs.
The deck itself is a frame: structure and headings from the real
src/apps/00-Template anatomy, content still to be written. Unfinished spots
are written as inline `TODO: ...` and rendered in red so a half-finished
deck cannot be presented by accident.
Also fixes the Docker fallback, which could not write to a mktemp directory
on macOS (not shared with Docker Desktop) and must not be passed --user.
