Collapse sidebar navigation list on smaller screens

- CSS only solution using "check-box hack"
- Add new UI text label `menu_label` in _data/ui-text.yml
- Close #607

_includes/nav_list

@@ -1,8 +1,10 @@
 {% assign navigation = site.data.navigation[include.nav] %}
 
 <nav class="nav__list">
-  {% if page.sidebar.title %}<header><h4 class="nav__title" style="padding: 0;">{{ page.sidebar.title }}</h4></header>{% endif %}
-  <ul>
+  {% if page.sidebar.title %}<h3 class="nav__title" style="padding-left: 0;">{{ page.sidebar.title }}</h3>{% endif %}
+  <input id="ac-toc" name="accordion-toc" type="checkbox" />
+  <label for="ac-toc">{{ site.data.ui-text[site.locale].menu_label | default: "Toggle Menu" }}</label>
+  <ul class="nav__items">
     {% for nav in navigation %}
       <li>
         {% if nav.url %}