getversion: handling detached checkouts
Makefile: doc/_build can be a symlink

Using 2 commands, since $(BUILDDIR) can be a symlink. Usually to a
subdir in $HOME/public_html/ to be able to debug it easily
Merge branch 'handle-detached-checkouts' into devel
doc: adding the preface summary
doc: introducting `m-r-b`
Merge branch 'guide' into devel