Finish moving How-tos to wiki

* The "How-tos" link on the sidebar will now go directly to http://wiki.dlang.org/Tutorials where the dlang.org how-tos have been migrated to.
* The "Tech Tips" at http://digitalmars.com/techtips/index.html have also all been migrated to http://wiki.dlang.org/Tutorials.
* The subnav under "How-tos" have all been removed as there is no way to display it since the "How-tos" link takes users directly to the wiki.
* Each individual how-to will display a link to the corresponding wiki page to prevent a 404 should a user follow an old link.