Phoenix docs: rename the `richtextctrl_overview.rst` with the underscore and add the snipped for `dataview.TreeListCtrl.GetNextItem.1.py`

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxPython/Phoenix/trunk@75889 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775