refactor: improve width handling and introduce CSS variables (#678)

* 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- Removed the outdated page-width utility partial to streamline the codebase.

* feat: introduce CSS variables for layout widths and update footer/navbar styles

- Added a new configs.css file to define CSS variables for page, navbar, and footer widths.
- Updated footer and navbar partials to utilize the new CSS classes for consistent width management.
- Refactored head-config-css.html to include the new navbar width variable.
- Enhanced the overall styling structure for better maintainability and responsiveness.

* Refactor: Rename configs.css to variables.css and update references

Remove head-config-css.html and update references to use variables.css instead of configs.css

* Update assets/css/variables.css

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Update layouts/partials/head.html

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

exampleSite/hugo.yaml

@@ -122,7 +122,7 @@ params:
     width: wide
 
   page:
-    # full (100%), wide (90rem), normal (1280px)
+    # full (100%), wide (90rem), normal (80rem)
     width: normal
 
   theme: