Prevent GitHub Pages action running on forks (#994)

Co-authored-by: Andy Waite <[email protected]>

Author: andyw8

.github/workflows/publish_docs.yml

@@ -7,6 +7,7 @@ on:
 jobs:
   build:
     runs-on: ubuntu-latest
+    if: github.repository == 'Shopify/ruby-lsp'
     name: Publish documentation website
     steps:
       - uses: actions/checkout@v3