Fix HTML validation issues (W3C validator) (#964)

Closes #963.

- redundant `role="navigation"` on nav elements
- redundant `type="text/javascript"` on script tags
- misplaced mermaid script tag

Note that [running the validator on the deploy preview](https://validator.w3.org/nu/?doc=https%3A%2F%2Fdeploy-preview-964--just-the-docs.netlify.app%2F) has no errors now!

_includes/head.html

@@ -27,14 +27,14 @@
   {% endif %}
 
   {% if site.search_enabled != false %}
-    <script type="text/javascript" src="{{ '/assets/js/vendor/lunr.min.js' | relative_url }}"></script>
+    <script src="{{ '/assets/js/vendor/lunr.min.js' | relative_url }}"></script>
   {% endif %}
 
   {% if site.mermaid %}
     <script src="https://cdn.jsdelivr.net/npm/mermaid@{{ site.mermaid.version }}/dist/mermaid.min.js"></script>
   {% endif %}
 
-  <script type="text/javascript" src="{{ '/assets/js/just-the-docs.js' | relative_url }}"></script>
+  <script src="{{ '/assets/js/just-the-docs.js' | relative_url }}"></script>
 
   <meta name="viewport" content="width=device-width, initial-scale=1">
 

_layouts/default.html

@@ -49,7 +49,7 @@ layout: table_wrappers
         <svg viewBox="0 0 24 24" class="icon"><use xlink:href="#svg-menu"></use></svg>
       </a>
     </div>
-    <nav role="navigation" aria-label="Main" id="site-nav" class="site-nav">
+    <nav aria-label="Main" id="site-nav" class="site-nav">
       {% assign pages_top_size = site.html_pages
           | where_exp:"item", "item.title != nil"
           | where_exp:"item", "item.parent == nil"
@@ -217,12 +217,13 @@ layout: table_wrappers
       <div class="search-overlay"></div>
     {% endif %}
   </div>
-</body>
-{% if site.mermaid %}
+
+  {% if site.mermaid %}
   <script>
     var config = {% include mermaid_config.js %};
     mermaid.initialize(config);
     window.mermaid.init(undefined, document.querySelectorAll('.language-mermaid'));
   </script>
-{% endif %}
+  {% endif %}
+</body>
 </html>