Implement new template system added with Hugo v0.146.0 (#1858)

* feat(layouts): move all files in layouts/_default up to the layouts/ root.

* feat(layouts): rename layouts/partials folder to layouts/_partials

* feat(layouts): rename layouts/shortcodes folder to layouts/_shortcodes

* feat(taxonomy): add new taxonomy layout template

A template named taxonomy.html used to be a candidate for both Page kind term and taxonomy, now it’s only considered for taxonomy.

layouts/_partials/social_icons.html

@@ -0,0 +1,8 @@
+<div class="social-icons" {{ with .align}}align="{{.}}" {{- end }}>
+    {{- range site.Params.socialIcons }}
+    <a href="{{ trim .url " " | safeURL }}" target="_blank" rel="noopener noreferrer me"
+        title="{{ (.title | default .name) | title }}">
+        {{ partial "svg.html" . }}
+    </a>
+    {{- end }}
+</div>