docs(experiments): document CSS scope hoisting and @value in URLs (5.107) (#8244)

Webpack 5.107 brings two additions to the experiments.css feature set:

- Scope hoisting (module concatenation) now applies to CSS Modules with
  exportType "text", "css-style-sheet", "style", or "link", reducing
  runtime overhead in CSS-heavy bundles when
  optimization.concatenateModules is enabled.
- @value identifiers can be used as the path argument to @import and
  inside url() references, so shared paths/assets are defined once and
  reused. Both quoted and bare forms are accepted and resolved through
  webpack's normal asset pipeline.

Adds two bullets to the existing experimental features list in
experiments.css.

Refs:
- webpack/webpack#20851 (scope hoisting)
- webpack/webpack#20925 (@value in URLs)

Author: bjohansebas

src/content/configuration/experiments.mdx

@@ -238,6 +238,31 @@ Experimental features:
 - CSS imports: webpack will inline CSS imports into the generated CSS file.
 - Hot Module Reload (HMR): webpack supports HMR for CSS files. This means that changes made to CSS files will be reflected in the browser without a full page reload.
 
+- <Badge text="5.107.0+" /> Scope hoisting (module concatenation) for CSS
+  Modules. When
+  [`optimization.concatenateModules`](/configuration/optimization/#optimizationconcatenatemodules)
+  is enabled, CSS Modules whose `exportType` is `text`, `css-style-sheet`,
+  `style`, or `link` are concatenated into a single module instance instead of
+  being kept as separate runtime instances. This reduces overhead and produces
+  smaller output for CSS-heavy bundles.
+
+- <Badge text="5.107.0+" /> `@value` identifiers can be used as the path
+  argument to `@import` and inside `url()` references, so shared paths and
+  assets can be defined once and reused across stylesheets. Both quoted
+  (`"./x"`, `'./x'`) and bare (`./x`) forms are accepted and resolved through
+  webpack's normal asset pipeline.
+
+  ```css
+  @value path: "./other.module.css";
+  @import path;
+
+  @value bg: "./image.png";
+
+  .a {
+    background: url(bg);
+  }
+  ```
+
 ### experiments.cacheUnaffected
 
 Enable additional in-memory caching of modules which are unchanged and reference only unchanged modules.