docs: register the pm/release developer pages in the sidebar

The five pages moved from docs/ into website/docs/developer-guide were
never added to sidebars.ts, so the site did not link them, and they kept
repo-relative links (../tests/..., ../scripts/..., ../website/docs/...)
that only resolved from the old docs/ location. Register them under a
Packaging & Releases category and point the repo-file links at GitHub;
package-management.md now links the moved completion note in place.

website/sidebars.ts

@@ -854,6 +854,17 @@ const sidebars: SidebarsConfig = {
             'developer-guide/trajectory-format',
           ],
         },
+        {
+          type: 'category',
+          label: 'Packaging & Releases',
+          items: [
+            'developer-guide/source-update-completion',
+            'developer-guide/shared-bundle-builds',
+            'developer-guide/stable-releases',
+            'developer-guide/macos-bundle-updates',
+            'developer-guide/pm-audit-status',
+          ],
+        },
       ],
     },
     {