Add condition on "deploy" step to only trigger on forks (#32)

see DISCLAIMER comment
add required secrets in comment

On main branch the condition on the deploy step was:
github.event_name == 'push' || github.event_name == 'workflow_dispatch'

It must be resolved given that we want to avoid triggering on forks (due
to possibly missing secrets leading to failing workflow)
it then become:
(github.event_name == 'push' && github.event.repository.fork == 'false') || github.event_name == 'workflow_dispatch'
    <==>
If it is a classic push:
 - on fork *do not* trigger deployment
 - on base repo trigger deployment
But if it is manually triggered, execute deployment step anyway
maybe we're on fork maybe we're not but this is user triggered so it the
user's responsability anyway.

.github/workflows/build_and_deploy.yml

@@ -39,10 +39,23 @@ jobs:
           path: ./build/blog
 
   # Deployment job: heavily inspired from https://swharden.com/blog/2022-03-20-github-actions-hugo/
-  # /!\ only triggers on push events and manually triggered
+  # /!\ only triggers on (push events AND non-fork repos) OR manually triggered
+  ## Required secrets:
+  #   - SSH_KNOWN_HOSTS
+  #   - PRIVATE_SSH_KEY
+  #   - CI_USER_NAME
+  #   - STATIC_WEBSITE_PATH
   deploy:
     needs: [build]
-    if: ${{ github.event_name == 'push' || github.event_name == 'workflow_dispatch' }}
+    # DISCLAIMER:
+    # The following is a very POOR solution to avoid *failing deploy step* due to missing secrets
+    # on fork repositories, but sadly the `env` context is not accessible from `jobs.<job_id>.if`:
+    # https://docs.github.com/en/actions/learn-github-actions/contexts#context-availability
+    #
+    # If for any reason you want to trigger this step on your fork remove the following line,
+    # trigger manually or open an issue https://github.com/iScsc/blog.iscsc.fr/issues,
+    # we'll find a better way to skip this step.
+    if: ${{ (github.event_name == 'push' && github.event.repository.fork == 'false') || github.event_name == 'workflow_dispatch' }}
     runs-on: ubuntu-latest
     steps:
       - name: 🛠️ Setup build directory