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/page/search.json

@@ -1,4 +1,4 @@
-{{- $pages := where .Site.RegularPages "Type" "in" .Site.Params.mainSections -}}
+{{- $pages := partial "helper/pages.html" .Site.Home -}}
 {{- $result := slice -}}
 
 {{- range $pages -}}