GH Actions: don't run the docs workflow on forks (#2371)

The `docs` workflow to deploy the GH Pages website is run on pushes to `master`, but that includes pushes to `master` in forks, which obviously can't deploy to the GH Pages site.

This means that in forks (and there are nearly 9000 of them), this workflow will always fail, while in reality, it shouldn't be run in the first place.

So, I'd like to propose making this small change, which _should_ prevent the `docs` workflow from being run on forks.

Co-authored-by: jrfnl <jrfnl@users.noreply.github.com>

.github/workflows/docs.yaml

@@ -7,6 +7,7 @@ jobs:
   build_and_publish:
     name: Build and publish Docs
     runs-on: ubuntu-latest
+    if: github.repository == 'PHPMailer/PHPMailer'
     steps:
       - name: Checkout sources
         uses: actions/checkout@v1