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/feature-grid.html

@@ -0,0 +1,12 @@
+{{- $cols := .Get "cols" | default 3 -}}
+{{- $style := .Get "style" | default "" -}}
+
+{{- $css := printf "--hextra-feature-grid-cols: %v; %s" $cols $style -}}
+
+
+<div
+  class="hextra-feature-grid hx:grid hx:sm:max-lg:grid-cols-2 hx:max-sm:grid-cols-1 hx:gap-4 hx:w-full not-prose"
+  {{ with $css }}style="{{ . | safeCSS }}"{{ end }}
+>
+  {{ .Inner }}
+</div>