[DOXIA-499][DOXIASITETOOLS-84] moved o.a.m.doxia.sink.render.RenderingContext from Doxia core to Doxia Site Renderer

git-svn-id: https://svn.apache.org/repos/asf/maven/doxia/doxia-sitetools/trunk@1534356 13f79535-47bb-0310-9956-ffa450edef68