fix: Refactor global variables

It's not possible to migrate away from `@import` because:

```
pnpx sass-migrator module --migrate-deps addon/styles/addon.scss

...

Error: This stylesheet was loaded by a nested import in addon/styles/addon.scss. The module system only supports loading nested CSS using the load-css() mixin, which doesn't allow access to variables from the outer stylesheet.
  ╷
2 │   width: calc(((100% - #{$site-container}) / 2) + 14rem);
  │                          ^^^^^^^^^^^^^^^
  ╵
  addon/styles/components/_docs-viewer-x-current-page-index.scss 2:26  root stylesheet
```

Author: MichalBryxi

addon/styles/_variables.scss

@@ -0,0 +1,2 @@
+// Global settings
+$site-container: 1400px;

addon/styles/addon.scss

@@ -1,6 +1,3 @@
-// Global settings
-$site-container: 1400px;
-
 @tailwind base;
 @tailwind components;
 @tailwind utilities;

addon/styles/components/_docs-viewer-x-current-page-index.scss

@@ -1,5 +1,7 @@
+@use '../variables' as vars;
+
 .AddonDocs-DocsViewer-CurrentPageIndex {
-  width: calc(((100% - #{$site-container}) / 2) + 14rem);
-  padding-right: calc((100% - #{$site-container}) / 2);
+  width: calc(((100% - #{vars.$site-container}) / 2) + 14rem);
+  padding-right: calc((100% - #{vars.$site-container}) / 2);
   min-width: 14rem;
 }