_content: show anchor links when hovering over article headers

Previously, users had to hover directly over the ΒΆ symbol to reveal the
anchor link, making it difficult to discover its existence.

This change updates the CSS to also display the anchor link when hovering
over the entire header text, improving usability.

Fixes golang/go#70827

Change-Id: I179bba16affc5dc79bb99dd74d6ea3ac7c2f375e
GitHub-Last-Rev: ebcdebd4c209d8cf6c99d47c95a0e587e850551b
GitHub-Pull-Request: golang/website#318
Reviewed-on: https://go-review.googlesource.com/c/website/+/663875
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Sean Liao <sean@liao.dev>
Reviewed-by: Sean Liao <sean@liao.dev>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: David Chase <drchase@google.com>
diff --git a/_content/css/styles.css b/_content/css/styles.css
index 254337c..673a8ab 100644
--- a/_content/css/styles.css
+++ b/_content/css/styles.css
@@ -875,6 +875,10 @@
 .Article a.Article-idLink {
   opacity: 0;
 }
+.Article h1:hover a.Article-idLink,
+.Article h2:hover a.Article-idLink,
+.Article h3:hover a.Article-idLink,
+.Article h4:hover a.Article-idLink,
 .Article a.Article-idLink:hover {
   opacity: 1;
   padding: 0.2rem;