anklang

misc/mkAppImage.sh: fetch linuxdeploy and appimage-runtime-zstd from upstream

Merge branch 'simplify-release-builds'

external/Makefile.mk: add json.hpp from nlohmann/json

doc/ch-releasing.md: update Release Assets section

misc/Makefile.mk, misc/mkassets.sh: remove obsolete asset build rule & script