add documentation for group layers/layer trees
add documentation for group layers/layer trees

--HG--
branch : 0.9.1.x
extra : transplant_source : %C5%D3%ED%D7%23%87%BB%00%E8%F4%AE%ED%FF%E2%3FN%3B%FB%87%7F

Anderson0026/mapproxy,drnextgis/mapproxy,vrsource/mapproxy,geoadmin/mapproxy,camptocamp/mapproxy,mapproxy/mapproxy,geoadmin/mapproxy,olt/mapproxy,mapproxy/mapproxy,kaiCu/mapproxy,procrastinatio/mapproxy,faegi/mapproxy,kaiCu/mapproxy,faegi/mapproxy,procrastinatio/mapproxy,camptocamp/mapproxy,drnextgis/mapproxy,olt/mapproxy,vrsource/mapproxy,Anderson0026/mapproxy