Remove yarn commands in documentations

Author: Kocal

CONTRIBUTING.md

@@ -13,8 +13,6 @@ Install the test app:
 
     $ composer install
     $ cd tests/app
-    $ yarn install
-    $ yarn build
     $ php public/index.php doctrine:schema:create
 
 Start the test app:

README.md

@@ -37,8 +37,6 @@ Configure test environment (working directory: `src/Turbo`):
 composer update
 docker compose up -d
 cd tests/app
-yarn install
-yarn build
 php public/index.php doctrine:schema:create
 ```
 

doc/index.rst

@@ -37,10 +37,6 @@ needed if you're using AssetMapper):
     $ npm install --force
     $ npm run watch
 
-    # or use yarn
-    $ yarn install --force
-    $ yarn watch
-
 Usage
 -----