chore: adapt theme to new template system (#696)

* Adapted theme's layout to [Hugo v0.146.0](https://gohugo.io/templates/new-templatesystem-overview/)
* Bumped minimal Hugo version to v0.146.0

layouts/_shortcodes/icon.html

@@ -0,0 +1,13 @@
+{{- $name := .Get "name" | default (.Get 0) -}}
+{{- $icon := index site.Data.icons $name -}}
+{{- $attributes := .Get "attributes" | default "height=1em"}}
+
+{{- if not $icon -}}
+  {{ errorf "icon %q not found" $name }}
+{{- end -}}
+
+{{- $icon = replaceRE "<svg" (printf "<svg %s" $attributes) $icon -}}
+
+<span class="hx:inline-block hx:align-text-bottom icon">
+  {{- $icon | safeHTML -}}
+</span>