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/hextra/hero-subtitle.html

@@ -0,0 +1,9 @@
+{{- $style := .Get "style" -}}
+
+
+<p
+  class="not-prose hx:text-xl hx:text-gray-600 hx:dark:text-gray-400 hx:sm:text-xl"
+  {{ with $style }}style="{{ . | safeCSS }}"{{ end }}
+>
+  {{ .Inner | markdownify }}
+</p>