Move template files based on Hugo v0.146 revamp

See https://gohugo.io/templates/new-templatesystem-overview/

Under layouts/
- mv _default/* .
- mv partials _partials
- mv shortcodes _shortcodes
- mv taxonomy/list.html taxonomy.html
- mv taxonomy/term.html term.html

layouts/_partials/top.html

@@ -0,0 +1,24 @@
+<div class="top">
+    <header class="header">
+        <a class="heading-link" href="{{ .Site.Home.Permalink | absLangURL }}">
+            {{ .Site.Title | markdownify }}
+        </a>
+    </header>
+
+    <nav class="navbar" aria-label="{{ i18n "aria_navbar" }}">
+        {{ with .Site.Home }}
+            <a class="navbar-item{{ if $.IsHome }} active{{ end }}" href="{{ .Permalink | absLangURL }}">{{ .Title | markdownify }}</a>
+        {{ end }}
+
+        {{ range .Site.Menus.main }}
+            {{ $isActive := false }}
+            {{/* https://discourse.gohugo.io/t/ismenucurrent-not-working-with-using-sectionpagesmenu/46687/2?u=bbaovanc */}}
+            {{ if or ($.HasMenuCurrent .Menu .) (eq .Page $) }}
+                {{ $isActive = true }}
+            {{ end }}
+            <a class="navbar-item{{ if $isActive }} active{{ end }}"
+               {{ if strings.HasPrefix .URL "http" }}target="_blank" rel="noopener"{{ end }}
+               href="{{ .URL }}">{{ .Name }}</a>
+        {{ end }}
+    </nav>
+</div>