Keep the old files to avoid breaking existing links to them, but just point to
the files in the new contributing subdirectory with more meaningful names now.
Also remove a couple of long obsolete tech notes.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76689 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
