refactor: centralize page filtering logic, and allow sort by lastmod (#1250)

* refactor: migrate page visibility control from `Params.hidden` to the new `build.list` parameter.

* refactor: centralize page filtering logic into a new `helper/pages.html` partial

* feat: introduce `SortBy` parameter to enable sorting pages by last modified date

* refactor: extract page sorting logic into a new `pages-sort.html` partial

layouts/_partials/helper/pages-sort.html

@@ -0,0 +1,7 @@
+{{- $pages := .Pages -}}
+
+{{- if eq .Context.Site.Params.SortBy "lastmod" -}}
+    {{- $pages = $pages.ByLastmod.Reverse -}}
+{{- end -}}
+
+{{- return $pages -}}