refactor: update page width handling and CSS structure

- Changed the default page width from 1280px to 80rem in hugo.yaml.
- Replaced dynamic page width partials with a new CSS class 'hextra-max-page-width' across multiple layout files for consistency.
- Introduced a new head-config-css.html partial for managing CSS styles related to page width.
- Removed the outdated page-width utility partial to streamline the codebase.

layouts/docs/list.html

@@ -1,5 +1,5 @@
 {{ define "main" }}
-  <div class='hx:mx-auto hx:flex {{ partial "utils/page-width" . }}'>
+  <div class='hx:mx-auto hx:flex hextra-max-page-width'>
     {{ partial "sidebar.html" (dict "context" .) }}
     {{ partial "toc.html" . }}
     <article class="hx:w-full hx:break-words hx:flex hx:min-h-[calc(100vh-var(--navbar-height))] hx:min-w-0 hx:justify-center hx:pb-8 hx:pr-[calc(env(safe-area-inset-right)-1.5rem)]">

layouts/docs/single.html

@@ -1,5 +1,5 @@
 {{ define "main" }}
-  <div class='hx:mx-auto hx:flex {{ partial "utils/page-width" . }}'>
+  <div class='hx:mx-auto hx:flex hextra-max-page-width'>
     {{ partial "sidebar.html" (dict "context" .) }}
     {{ partial "toc.html" . }}
     <article class="hx:w-full hx:break-words hx:flex hx:min-h-[calc(100vh-var(--navbar-height))] hx:min-w-0 hx:justify-center hx:pb-8 hx:pr-[calc(env(safe-area-inset-right)-1.5rem)]">