Restrict CI on forks.

- Don't do Windows CI on forks. (We already don't do Linux CI on forks.)

- Don't do container image CI on forks. (We shouldn't be pushing
  container images to ghcr.io from forks!)

- Remove the `if` condition in `deploy_guide.yml`. It has no effct
  because there is an `on: push: branches: - main` condition higher up
  doing the same thing (and in the more typical way).

Author: nnethercote

.github/workflows/ci_windows.yml

@@ -5,6 +5,8 @@ on:
     paths-ignore:
       - "**.md"
   push:
+    branches:
+      - main
     paths-ignore:
       - "**.md"