Each major target of polybar can now be enabled/disabled while
configuring (even polybar itself).
The cmake code specific to each target will only run if the target is
enabled.
This allows us to for example just build the documentation without
having to run all the cmake code related to compilation or having the
polybar dependencies installed (other than sphinx).
* Adopt keep a changelog
Ref: https://keepachangelog.com/en/1.0.0/
* Add changelog to release and contributing docs
* Use H2 for automatically added Download section
The individual changelog subsections use H3 and the changelog section
uses H2, so the Download section should use the same heading
* Add already present changes to changelog
* Mention changelog issue references in CONTRIBUTING.md
Whenever a release is published, this action will create a release
archive and update the release body with a download section.
This also integrates the suggestions from #2276Resolves#2276
