docs: add tasks regarding socket activation
units: add newline at the end of file
units: introduce instanced.target

Make possible to stop all instances of instanced service using target.
docs: add solutions to socket activation tasks
build-sys: fix installation of tasks and solutions

tasks.txt and solutions.txt now reside in docs/ folder. We should reflect this
change in Makefile too.
spec: mark tasks.txt and solutions.txt as documentation