Add first interaction workflow to direct contributors to contributing guidelines

Author: msimberg

.github/workflows/welcome.yaml

@@ -0,0 +1,21 @@
+name: First interaction message
+
+on:
+  pull_request_target:
+    types: [opened]
+    branches:
+      - main
+
+jobs:
+  greeting:
+    runs-on: ubuntu-latest
+    permissions:
+      pull-requests: write
+    steps:
+    - uses: actions/first-interaction@v3
+      with:
+        pr-message: |
+          Thank you for your contribution to eth-cscs/cscs-docs.
+
+          If you have not done so already, please take some time to get familiar with the [contributing guidelines](https://docs.cscs.ch/contributing/).
+          Following the guidelines helps us keep the documentation consistent and as useful as possible for users.