- Move doc of Triangulation_utils_3 to TDS chapter.
- Document new concept TriangulationHierarchyVertexBase_3.
- Various doc improvements.
- Add Triangulation_vertex_base_with_info<Info, GT, Vb>.
- Add Triangulation_cell_base_with_info<Info, GT, Cb>.
- Add create_vertex(Vertex_handle), and document it as well as create_cell(Ch).