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_pack_counts.py

@@ -536,8 +536,8 @@ class TheReleaseNotesReadTheRecord(unittest.TestCase):
         text = self.render()
         row = next(line for line in text.splitlines() if line.startswith("| RetroArch"))
         self.assertIn("8,504", row)
-        self.assertIn("part1of2.zip", row)
-        self.assertIn("part2of2.zip", row)
+        self.assertIn("[Download, 2 parts](https://abdess.github.io/retrobios/get/retroarch/)", row)
+        self.assertNotIn("/releases/download/", row)
         self.assertIn("`system/`", row)
         self.assertIn("| Platform | Download | Extracted size | Files |", text)