Transition from NUS Downloader to NUSGet

[grunt22fox]

NUS Downloader is the current application referenced in the guide for obtaining resources from NUS. As referenced in another pull request, this program is outdated and depreciated. This PR seeks to transition to NUSGet, a more user friendly and actively maintained application for the same purpose.

TODO:

Change references in bricks.md ✅ Change references in cios.md ✅ Change references in themes.md ✅ Change references in update.md ✅

[mpgiii]

Is there any reason we haven't had any more movement on this PR? Would love to get this transition to NUSGet in.