* update build instructions
Usage of "modern" CMake syntax and using specific out of source build.
Further add example on how to build in parallel, build a specific target
and how to change the compiler.
This closes#3236.
* move doc/misc to top level, add READMEs
* Move docs -> doc/devel
This also consolidates the _three_ documents describing (differently)
how to build Taskwarrior into a signle document.