docs: polish search form

docs/_ext/templates/search.html

@@ -27,24 +27,35 @@
     data-no-results="{{ '没有找到匹配结果。' if is_zh else 'No matching results found.' }}"
     data-result-one="{{ '找到 1 个匹配结果。' if is_zh else 'Found 1 matching result.' }}"
     data-result-many="{{ '找到 {count} 个匹配结果。' if is_zh else 'Found {count} matching results.' }}"
+    data-history-title="{{ '最近搜索' if is_zh else 'Recent searches' }}"
+    data-history-empty="{{ '还没有搜索记录。' if is_zh else 'No recent searches yet.' }}"
+    data-clear-history="{{ '清空历史' if is_zh else 'Clear history' }}"
   >
     <p>
       {{ '支持中文和英文搜索，结果会优先跳转到最接近的章节位置。' if is_zh else 'Supports both Chinese and English search and jumps to the closest matching section.' }}
     </p>
 
-    <form action="" method="get" role="search">
-      <input
-        type="text"
-        name="q"
-        aria-labelledby="search-documentation"
-        value=""
-        autocomplete="off"
-        autocorrect="off"
-        autocapitalize="off"
-        spellcheck="false"
-      />
-      <input type="submit" value="{{ _('search') }}" />
-    </form>
+    <div class="section-search-panel">
+      <form action="" method="get" role="search" class="section-search-form">
+        <div class="section-search-form__field">
+          <span class="section-search-form__icon" aria-hidden="true">⌕</span>
+          <input
+            type="text"
+            name="q"
+            aria-labelledby="search-documentation"
+            value=""
+            placeholder="{{ '输入关键词，例如：相机、sync_mode、网络相机' if is_zh else 'Search keywords like camera, sync_mode, network camera' }}"
+            autocomplete="off"
+            autocorrect="off"
+            autocapitalize="off"
+            spellcheck="false"
+          />
+        </div>
+        <button type="submit" class="section-search-form__submit">{{ _('search') }}</button>
+      </form>
+
+      <div id="section-search-history" class="section-search-history" hidden></div>
+    </div>
 
     <p id="section-search-status" class="search-summary"></p>
     <div id="section-search-results"></div>