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).
This also moves the doc generation completely into cmake (no more
Makefile).
To generate the docs the project needs to first be configured and then
`make doc` can be run.
The approach used is leaned on the cmake's project own use of Sphinx:
Utilities/Sphinx/CMakeLists.txt
