minor #1062 [Site] Using symfonycasts/sass-bundle for sass processing (weaverryan)

This PR was squashed before being merged into the 2.x branch.

Discussion
----------

[Site] Using symfonycasts/sass-bundle for sass processing

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| Tickets       | None
| License       | MIT

Hi!

https://github.com/SymfonyCasts/sass-bundle allows us to build the Sass with just a single command - and the built file no longer needs to be committed 🎆

Cheers!

Commits
-------

fb83402 [Site] Using symfonycasts/sass-bundle for sass processing

Author: weaverryan

.github/workflows/ux.symfony.com.yaml

@@ -24,5 +24,8 @@ jobs:
               with:
                 working-directory: ux.symfony.com
 
+            - name: Build Sass assets
+              run: php bin/console sass:build
+
             - name: Tests
               run: vendor/bin/phpunit

ux.symfony.com/.platform.app.yaml

@@ -49,6 +49,7 @@ hooks:
 
         (>&2 symfony-build)
 
+        php bin/console sass:build
         php bin/console asset-map:compile
 
     deploy: |

ux.symfony.com/README.md

@@ -28,6 +28,11 @@ Source code for [ux.symfony.com](https://ux.symfony.com).
     symfony server:start -d
     ```
 
+6. Compile the Sass files:
+    ```bash
+    php bin/console sass:build --watch
+    ```
+
 ### Running the Test Suite
 
 ```bash