[rpm] provider-doc requires graphviz

Now need to add BuildRequires under _with_docs conditional to allow to
build docs with diagrams if graphviz is available, this is not true for
nemo now, so there should be ability to generate docs w/o
diagrams. Later (when graphviz will be available) provider-doc package
generation can be also put under _with_docs conditional.

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
[examples] documenting statefspp-based example

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>