fix: skip docs deployment workflow on forks (#2171)

Add repository check to build and deploy jobs so they skip
with success on forks, avoiding failed Pages deployments for
contributors.

.github/workflows/docs.yml

@@ -26,6 +26,7 @@ concurrency:
 jobs:
   # Build job
   build:
+    if: github.repository == 'github/spec-kit'
     runs-on: ubuntu-latest
     steps:
       - name: Checkout
@@ -56,6 +57,7 @@ jobs:
 
   # Deploy job
   deploy:
+    if: github.repository == 'github/spec-kit'
     environment:
       name: github-pages
       url: ${{ steps.deployment.outputs.page_url }}