refactor(layout): keep content width fixed and document CSS var override (#934)

* fix(layout): split page and content width configuration

* refactor(layout): consolidate page width settings and update documentation

- Removed separate content width configuration from `hugo.yaml` and set a default value of `72rem` in CSS.
- Updated documentation in multiple languages to reflect the changes in page and content width settings, emphasizing the new fixed content width and customization options.

layouts/_partials/utils/page-width-override.html

@@ -1,7 +1,10 @@
 {{- with .Params.width -}}
+  {{- $pageWidthValues := dict "normal" "80rem" "wide" "90rem" "full" "100%" -}}
+  {{- $pageWidth := . -}}
+  {{- $maxPageWidth := (index $pageWidthValues $pageWidth) | default (index $pageWidthValues "normal") -}}
 <style>
 :root {
-  --hextra-max-page-width: {{ cond (eq . "wide") "90rem" (cond (eq . "full") "100%" "80rem") }};
+  --hextra-max-page-width: {{ $maxPageWidth }};
 }
 </style>
 {{- end -}}