readthedocs: Also fetch all other branches

'stable/2.0' is a branch, not a tag.

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

Author: stephenfin

.readthedocs.yaml

@@ -10,4 +10,4 @@ build:
   jobs:
     post_checkout:
       - git fetch --unshallow
-      - git fetch --tags
+      - git fetch --all --tags