feat: guide a stale link and the first download

Readers reached the 404 page with a pasted command (/i.ps1%20%7C%20iex),
a file name, a GitHub path or a section below a page: the page now says
which it is and leads on. Beside "Start with", the first file's
download sits next to its name.

scripts/generate_site.py

@@ -11584,7 +11584,7 @@ def generate_not_found() -> str:
         "- Browse [platforms](platforms/index.md), [systems](systems/index.md) "
         "or [emulators](emulators/index.md)",
         "",
-        '<p id="rb-notfound-hint" hidden></p>',
+        '<div id="rb-notfound-hint" hidden></div>',
         "",
     ])