readthedocs: Fetch all branches

Third time lucky. As this is configured, we were only fetching the main
branch. Correct this.

Signed-off-by: Stephen Finucane <stephen@that.guru>

.readthedocs.yaml

@@ -10,4 +10,5 @@ build:
   jobs:
     post_checkout:
       - git fetch --unshallow
+      - git config remote.origin.fetch '+refs/heads/*:refs/remotes/origin/*'
       - git fetch --all --tags