🤖 docs: Simplify scripts.

Replace querySelectorAll(...)[0] by querySelector(...) in docs scripts.

These changes were automatically generated by a transform whose code can be found at:
  - https://github.com/make-github-pseudonymous-again/rejuvenate/blob/d09fbf6436be892b61bbe02454e1d7d7229c50d8/src/transforms/codemod:doc-scripts-header-replace-querySelectorAll.js
Please contact the author of the transform if you believe there was an error.

doc/scripts/header.js

@@ -13,7 +13,7 @@ domReady(() => {
 	projectname.text = '@union-find/non-contiguous';
 	projectname.href = './index.html';
 
-	const header = document.querySelectorAll('header')[0];
+	const header = document.querySelector('header');
 	header.insertBefore(projectname, header.firstChild);
 
 	const testlink = document.querySelector('header > a[data-ice="testLink"]');