ci: the documentation is deployed from master alone

A manual run against a branch built the site and then published it, because the deploy job followed the build with no
condition on the ref. The build is the part worth running from a branch; the publish is not.

Author: MrRefactoring

.github/workflows/docs.yml

@@ -56,6 +56,7 @@ jobs:
   deploy:
     name: 🚀 Deploy to GitHub Pages
     needs: build
+    if: github.ref == 'refs/heads/master'
     runs-on: ubuntu-latest
     environment:
       name: github-pages