feat: permanent download page per platform

get/<platform>/ lists the files of the current release pack, read from
release.json, or gives the command for a setup without one. The README,
setup pages, Get your files and the release notes link it instead of
asset URLs, which die with each deleted tag. Setups without a ZIP say so
with the command beside it, and validate_site checks every get/ page.

tests/test_minui_platform.py

@@ -429,7 +429,9 @@ class TheSitePageOfACardPlatform(unittest.TestCase):
         self.assertNotIn("0/0", page)
         self.assertNotIn("## Systems overview", page)
         self.assertIn("| `Bios/PS/` | [pcsx_rearmed](../emulators/pcsx_rearmed.md) |", page)
-        self.assertIn("| Unzip the pack into | `<SD card>/` |", page)
+        # MinUI has no release pack: no unzip row, the command instead.
+        self.assertNotIn("| Unzip the pack into |", page)
+        self.assertIn("No ZIP for MinUI. The command installs every file and checks it.", page)
 
 
 if __name__ == "__main__":