Make: `doc` generates example config
Make: `doc` generates example config

This means that once `make all` finished, a call to `make
install-examples` won't try to build anything.

Issue spotted by macports maintainers:
https://github.com/macports/macports-ports/pull/7053#discussion_r422622159

newsboat/newsboat,der-lyse/newsboat,newsboat/newsboat,der-lyse/newsboat,der-lyse/newsboat,der-lyse/newsboat,newsboat/newsboat,der-lyse/newsboat,newsboat/newsboat,der-lyse/newsboat,newsboat/newsboat,der-lyse/newsboat,newsboat/newsboat,der-lyse/newsboat,newsboat/newsboat,newsboat/newsboat