feat: split the heavy site pages

A manufacturer page over 500 KB of Markdown becomes a summary: its
largest consoles move to pages of their own, cut into parts when one
console is still too large, and every console keeps its heading there.
MAME's 1,564 cards move to four pages, its page keeping a row per entry
with the card's old anchor. RetroDECK's arcade list gets its own page.
site.js follows a moved card by content or by its row.

The largest page drops from 10.5 MB to 0.88 MB. Emulator groups of the
navigation open by initial (65 KB to 41 KB before the content). File
cards are level 3 under a level 2, and the hook keeps the table of
contents of card pages to level 2: platform pages no longer list every
system id. Key/value cards name their columns. The sweep runs before
the decoration, which kept every page no generator wrote any more.

docs_assets/site.js

@@ -238,10 +238,20 @@
   };
 
   // A file card moved to another page keeps its address: an id of the form
-  // sha1-<12 hex> that this page does not hold is looked up by content.
+  // sha1-<12 hex> that this page does not hold is looked up by content, and
+  // an emulator card listed in the table of a split page opens its page.
   const followMovedCard = () => {
-    const match = /^#sha1-([0-9a-f]{12})$/.exec(window.location.hash);
-    if (!match || document.getElementById(`sha1-${match[1]}`)) return;
+    const id = decodeURIComponent(window.location.hash.slice(1));
+    if (!id) return;
+    const target = document.getElementById(id);
+    const row = target?.closest(".rb-card-index tr");
+    const link = row?.querySelector("a[href]");
+    if (link) {
+      window.location.replace(link.href);
+      return;
+    }
+    const match = /^sha1-([0-9a-f]{12})$/.exec(id);
+    if (!match || target) return;
     window.location.replace(new URL(`files/?sha1=${match[1]}`, siteRoot()));
   };