Add tabix to doc build action

No python tabix writer available. This can be removed when we have a tskit2zarr utility.

.github/workflows/docs.yml

@@ -42,6 +42,7 @@ jobs:
         run: |
           python -m venv venv
           . venv/bin/activate
+          conda install -c bioconda tabix
           pip install --upgrade pip wheel
           pip install -r requirements/CI-docs/requirements.txt