pkg docs: add a tag for the "how to develop" section

Merge to v5.3.4
move `handin-client' and `handin-server' to `handin' package
raco setup: remove any extra documentation dirs in installation

After uninstalling a package in installation scope, its documentation
should be removed. A new `raco setup' step takes care of that.

Merge to v5.3.4
raco pkg: fix `raco setup' sensitivity to package scope

Merge to v5.3.4