doc: use semantically appropriate tag for lines

The HTML <hr> element represents a thematic break between
paragraph-level elements: for example, a change of scene in a story,
or a shift of topic within a section.
The HTML Content Division element (<div>) is the generic container
for flow content and does not inherently represent anything.

The <hr> tag semantically fits better the meaning of the line between
the lists.

Refs: https://html.spec.whatwg.org/multipage/grouping-content.html#the-hr-element

PR-URL: #34660
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

doc/api/index.md

@@ -8,7 +8,7 @@
 * [About this documentation](documentation.html)
 * [Usage and example](synopsis.html)
 
-<div class="line"></div>
+<hr class="line"/>
 
 * [Assertion testing](assert.html)
 * [Async hooks](async_hooks.html)
@@ -63,6 +63,6 @@
 * [Worker threads](worker_threads.html)
 * [Zlib](zlib.html)
 
-<div class="line"></div>
+<hr class="line"/>
 
 * [Code repository and issue tracker](https://github.com/nodejs/node)

doc/api_assets/style.css

@@ -524,7 +524,7 @@ a code {
 
 #column2 .line {
   margin: 0 .5rem;
-  background-color: #707070;
+  border-color: #707070;
 }
 
 #column2 ul li:last-child {