doc(doc/lua): add universe level documentation
doc(doc/lua): add universe level documentation

Signed-off-by: Leonardo de Moura <7610bae85f2b530654cc716772f1fe653373e892@microsoft.com>

Kha/lean,levnach/lean,leanprover-community/lean,fpvandoorn/lean,UlrikBuchholtz/lean,digama0/lean,leodemoura/lean,digama0/lean,sp3ctum/lean,sp3ctum/lean,fpvandoorn/lean2,htzh/lean,UlrikBuchholtz/lean,soonhokong/lean-windows,digama0/lean,eigengrau/lean,Kha/lean,javra/lean,leanprover-community/lean,fpvandoorn/lean,javra/lean,avigad/lean,UlrikBuchholtz/lean,eigengrau/lean,leanprover-community/lean,digama0/lean,johoelzl/lean,Kha/lean,leanprover/lean,soonhokong/lean,soonhokong/lean,rlewis1988/lean,eigengrau/lean,leodemoura/lean,Kha/lean,Kha/lean,dselsam/lean,htzh/lean,fgdorais/lean,soonhokong/lean-osx,soonhokong/lean-windows,rlewis1988/lean,leanprover/lean,rlewis1988/lean,johoelzl/lean,rlewis1988/lean,soonhokong/lean-osx,avigad/lean,soonhokong/travis_test,soonhokong/travis_test,dselsam/lean,fpvandoorn/lean,UlrikBuchholtz/lean,c-cube/lean,levnach/lean,digama0/lean,leanprover-community/lean,htzh/lean,c-cube/lean,levnach/lean,dselsam/lean,leodemoura/lean,fgdorais/lean,leanprover-community/lean,levnach/lean,leodemoura/lean,UlrikBuchholtz/lean,leodemoura/lean,soonhokong/lean,rlewis1988/lean,dselsam/lean,fgdorais/lean,leodemoura/lean,johoelzl/lean,fgdorais/lean,fpvandoorn/lean,c-cube/lean,soonhokong/lean-osx,leanprover/lean,javra/lean,fpvandoorn/lean2,leanprover/lean,javra/lean,c-cube/lean,levnach/lean,digama0/lean,sp3ctum/lean,eigengrau/lean,dselsam/lean,dselsam/lean,c-cube/lean,soonhokong/lean-windows,eigengrau/lean,Kha/lean,soonhokong/lean,fgdorais/lean,htzh/lean,fpvandoorn/lean,fgdorais/lean,avigad/lean,soonhokong/travis_test,htzh/lean,soonhokong/lean,leanprover-community/lean,fpvandoorn/lean,johoelzl/lean,soonhokong/lean-osx,fpvandoorn/lean2,soonhokong/lean-osx,johoelzl/lean,soonhokong/lean-windows,sp3ctum/lean,leanprover/lean,avigad/lean,javra/lean,fpvandoorn/lean2,leanprover/lean,rlewis1988/lean,avigad/lean,fpvandoorn/lean2,soonhokong/travis_test,sp3ctum/lean,soonhokong/lean-windows,avigad/lean,johoelzl/lean