tools,doc: add "legacy" badge in the TOC

PR-URL: #37949
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>

Author: aduh95

doc/api_assets/style.css

@@ -515,6 +515,14 @@ hr {
   padding: 1px 3px;
   border-radius: 3px;
 }
+#toc .stability_3::after {
+  background-color: var(--blue1);
+  color: var(--white);
+  content: "legacy";
+  margin-left: .25rem;
+  padding: 1px 3px;
+  border-radius: 3px;
+}
 
 #apicontent li {
   margin-bottom: .5rem;

tools/doc/html.js

@@ -234,9 +234,9 @@ function preprocessElements({ filename }) {
           const [, prefix, number, explication] =
             text.value.match(STABILITY_RE);
 
-          const isStabilityIndex =
-            index - 2 === headingIndex || // General.
-            index - 3 === headingIndex;   // With api_metadata block.
+          // Stability indices are never more than 3 nodes away from their
+          // heading.
+          const isStabilityIndex = index - headingIndex <= 3;
 
           if (heading && isStabilityIndex) {
             heading.stability = number;