Med: doc+build: make shared chapters a proper dependency

...meaning following:

1. edit of shared chapter(s) -> rebuild docs using it
2. edit of shared chapter(s) -> asciidoc chapters internal to documents from 1. have to be rebuild (converted) after shared chapters and before the overall target document is built

Point #2 is because otherwise we end up in situation that chapters
internal to documents from 1. will cache the old version of included
shared chapter(s) in its XML form and hence the target document will
be built unchanged.

Additionally, consider byproducts related to shared chapters when
cleaning.

Signed-off-by: Jan Pokorný <jpokorny@redhat.com>