Use more specific selector for name links (go-gitea#29679)

Followup go-gitea#29305. As per discussion
in go-gitea#29666 (comment),
make this selector only search in the current `.markup` document, as
there can be multiples displayed at the same time.

@DanielMatiasCarvalho maybe you can review.

Author: silverwind

web_src/js/markup/anchors.js

@@ -39,7 +39,7 @@ export function initMarkupAnchors() {
     if (!href.startsWith('#user-content-')) continue;
     const originalId = href.replace(/^#user-content-/, '');
     a.setAttribute('href', `#${encodeURIComponent(originalId)}`);
-    if (document.getElementsByName(originalId).length !== 1) {
+    if (a.closest('.markup').querySelectorAll(`a[name="${originalId}"]`).length !== 1) {
       a.addEventListener('click', (e) => {
         scrollToAnchor(e.currentTarget.getAttribute('href'), false);
       });