generate the CSS file in build directory

[Roger-luo]

currently, we generate CSS files in src/assets, this is causing LiveServer falls into recursive execution of the make.jl script because it will watch the generated CSS file. One will need to ignore this CSS file in order to make LiveServer work correctly with this package. I'm not sure what would be the best way to solve this, but generate the CSS to the build directory might be the solution. But then the problem becomes how to make Documenter recognize this.