[SYCL][Doc] Disable GitHub Pages job in forks (#2513)

This change prevents GitHub Pages documentation job from running in
forked repositories.

Author: Alexander Batashev

.github/workflows/gh_pages.yml

@@ -7,6 +7,7 @@ on:
 jobs:
   build:
     runs-on: ubuntu-latest
+    if: github.repository == 'intel/llvm'
     steps:
     - uses: actions/checkout@v2
       with: