refactor: add standardized spacing variables and more style cleanup (#1287)

* refactor: add standardized spacing variables and more style cleanup

* refactor: rename `spacing-xxl` to `spacing-2xl` and introduce `spacing-3xl` for consistent spacing variables.

* refactor: Migrate related content styling to use the `subsection-list` component with updated layout adjustments.

* style: adjust padding values for article list tiles.

* refactor: Introduce shadow variables and dynamic spacing to prevent shadow cutoff in list layouts.

* refactor: relocate dark scheme Mermaid variables within the root scope.

layouts/_partials/article/components/related-content.html

@@ -1,9 +1,9 @@
 {{ $related := (.Site.RegularPages.Related .) | first 5 }}
 {{ with $related }}
-<aside class="related-content--wrapper">
+<aside>
     <h2 class="section-title">{{ T "article.relatedContent" }}</h2>
-    <div class="related-content">
-        <div class="flex article-list--tile">
+    <div class="subsection-list">
+        <div class="article-list--tile">
             {{ range . }}
                 {{ partial "article-list/tile" (dict "context" . "size" "250x150" "Type" "articleList") }}
             {{ end }}