docs: flatten Private Multisig out of Solutions category

The SOLUTIONS section header already groups these items, so the
extra Solutions category wrapper was redundant.

Author: BrianSeong99

sidebars.ts

@@ -68,8 +68,11 @@ const sidebars: SidebarsConfig = {
     },
     {
       type: "category",
-      label: "Solutions",
-      items: [{ type: "autogenerated", dirName: "builder/solutions" }],
+      label: "Private Multisig",
+      link: { type: "doc", id: "builder/solutions/private-multisig/index" },
+      items: [
+        { type: "autogenerated", dirName: "builder/solutions/private-multisig" },
+      ],
     },
 
     // ── REFERENCE