fix(docs): allow balanced parens in markdown link URLs

Previously the link-checker regex stopped at the first ')',
breaking on Next.js route-group paths like `app/(app)/...`. The
new regex matches one level of balanced parens inside the URL.

Caught by CI on PR #188 — pre-existing breakage from PBI-78 plan
doc that was already merged on main.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

scripts/check-doc-links.mjs

@@ -49,7 +49,9 @@ function headingSlugs(filePath) {
   return slugs;
 }
 
-const LINK_RE = /\[(?:[^\]]*)\]\(([^)]+)\)/g;
+// Match `[label](url)` where url may contain one level of balanced parens
+// (e.g. Next.js route groups like `app/(app)/...`).
+const LINK_RE = /\[(?:[^\]]*)\]\(((?:[^()]+|\([^()]*\))+)\)/g;
 
 function checkFile(filePath) {
   const content = readFileSync(filePath, 'utf8');