merge docs to validate_project workflow

.github/workflows/docs.yaml

@@ -1,19 +0,0 @@
-name: Docs
-
-on:
-    pull_request: null
-    push:
-        branches:
-            - master
-
-jobs:
-    docs:
-        runs-on: ubuntu-latest
-        steps:
-            -   uses: actions/checkout@v2
-            -   uses: shivammathur/setup-php@v1
-                with:
-                    php-version: 7.2
-                    coverage: none # disable xdebug, pcov
-            -   run: composer install --no-progress
-            -   run: composer check-docs

.github/workflows/validate_project.yaml

@@ -25,3 +25,5 @@ jobs:
             -   run: bin/rector sync-annotation-parser --dry-run --ansi
 
             -   run: bin/rector validate-services-in-sets --ansi
+
+            -   run: composer check-docs