Simplify image mirrors instructions

There is no need to repeat the full example here, simply state what
variables are.

docs/operations/mirror.md

@@ -7,6 +7,7 @@ The public mirror is useful to make the public resources download quickly in som
 You can follow the [offline](offline-environment.md) to config the image/file download configuration to the public mirror site. If you want to download quickly in China, the configuration can be like:
 
 ```shell
+# this should be in <your_inventory>/group_vars/k8s_cluster.yml
 gcr_image_repo: "gcr.m.daocloud.io"
 kube_image_repo: "k8s.m.daocloud.io"
 docker_image_repo: "docker.m.daocloud.io"
@@ -19,45 +20,6 @@ files_repo: "https://files.m.daocloud.io"
 Use mirror sites only if you trust the provider. The Kubespray team cannot verify their reliability or security.
 You can replace the `m.daocloud.io` with any site you want.
 
-## Example Usage Full Steps
-
-You can follow the full steps to use the kubesray with mirror. for example:
-
-Install Ansible according to Ansible installation guide then run the following steps:
-
-```shell
-# Copy ``inventory/sample`` as ``inventory/mycluster``
-cp -rfp inventory/sample inventory/mycluster
-
-# Update Ansible inventory file with inventory builder
-declare -a IPS=(10.10.1.3 10.10.1.4 10.10.1.5)
-CONFIG_FILE=inventory/mycluster/hosts.yaml python3 contrib/inventory_builder/inventory.py ${IPS[@]}
-
-# Use the download mirror
-cp inventory/mycluster/group_vars/all/offline.yml inventory/mycluster/group_vars/all/mirror.yml
-sed -i -E '/# .*\{\{ files_repo/s/^# //g' inventory/mycluster/group_vars/all/mirror.yml
-tee -a inventory/mycluster/group_vars/all/mirror.yml <<EOF
-gcr_image_repo: "gcr.m.daocloud.io"
-kube_image_repo: "k8s.m.daocloud.io"
-docker_image_repo: "docker.m.daocloud.io"
-quay_image_repo: "quay.m.daocloud.io"
-github_image_repo: "ghcr.m.daocloud.io"
-files_repo: "https://files.m.daocloud.io"
-EOF
-
-# Review and change parameters under ``inventory/mycluster/group_vars``
-cat inventory/mycluster/group_vars/all/all.yml
-cat inventory/mycluster/group_vars/k8s_cluster/k8s-cluster.yml
-
-# Deploy Kubespray with Ansible Playbook - run the playbook as root
-# The option `--become` is required, as for example writing SSL keys in /etc/,
-# installing packages and interacting with various systemd daemons.
-# Without --become the playbook will fail to run!
-ansible-playbook -i inventory/mycluster/hosts.yaml  --become --become-user=root cluster.yml
-```
-
-The above steps are by adding the "Use the download mirror" step to the [README.md](../README.md) steps.
-
 ## Community-run mirror sites
 
 DaoCloud(China)