docs: Add ‹› navigation buttons (#29461)

While visually unintrusive, these navigation links enable proper
navigation in readers and extensions like Vimium that rely on
rel=next/prev

Release Notes:

- N/A

docs/theme/css/chrome.css

@@ -177,6 +177,36 @@ a > .hljs {
   display: none;
 }
 
+.nav-wrapper {
+  margin-block-start: 50px;
+  display: block;
+}
+
+.nav-buttons {
+  display: flex;
+  justify-content: space-between;
+  align-items: center;
+  margin: 20px 0;
+  padding: 0 10px;
+}
+
+.nav-button {
+  align-items: center;
+  padding: 8px 12px;
+  background-color: var(--bg);
+  color: var(--fg) !important;
+  text-decoration: none;
+}
+
+.nav-button:hover {
+  background-color: var(--theme-hover);
+  color: var(--icons-hover) !important;
+}
+
+.nav-button i {
+  padding: 0 6px;
+}
+
 .mobile-nav-chapters {
   font-size: 2.5em;
   text-align: center;

docs/theme/index.hbs

@@ -200,6 +200,23 @@
                       </div>
                       {{{ content }}}
                     </main>
+
+                    <!-- Navigation arrows -->
+                    <div class="nav-wrapper">
+                      <div class="nav-buttons">
+                        {{#previous}}
+                          <a rel="prev" href="{{link}}" class="nav-button" title="{{title}}" aria-label="Previous chapter">
+                            <i class="fa fa-angle-left"></i>
+                          </a>
+                        {{/previous}}
+
+                        {{#next}}
+                          <a rel="next" href="{{link}}" class="nav-button" title="{{title}}" aria-label="Next chapter">
+                            <i class="fa fa-angle-right"></i>
+                          </a>
+                        {{/next}}
+                      </div>
+                    </div>
                 </div>
             </div>
         </div>