Add a description for environametal variable used in cluster create template

[Karthik-K-N]

/kind feature
/area provider/ibmcloud

Describe the solution you'd like
[A clear and concise description of what you want to happen.]

In the CAPIBM book, In cluster create template for both VPC and PowerVS we use number of environmetnal variables to pass values to the cluster create template, As a new user it is difficult to understand what values needs to passed to each of them, It would be good if it has some description and point to location in IBMCloud to find them.

Example for PowerVS template

IBMPOWERVS_SSHKEY_NAME="my-pub-key" \
IBMPOWERVS_VIP="192.168.167.6" \
IBMPOWERVS_VIP_EXTERNAL="163.68.65.6" \
IBMPOWERVS_VIP_CIDR="29" \
IBMPOWERVS_IMAGE_NAME="capibm-powervs-centos-streams8-1-26-2" \
IBMPOWERVS_SERVICE_INSTANCE_ID="3229a94c-af54-4212-bf60-6202b6fd0a07" \
IBMPOWERVS_NETWORK_NAME="capi-test" \
IBMACCOUNT_ID="ibm-accountid" \
IBMPOWERVS_REGION="osa" \
IBMPOWERVS_ZONE="osa21" \
BASE64_API_KEY=$(echo -n $IBMCLOUD_API_KEY | base64) \
clusterctl generate cluster ibm-powervs-1 --kubernetes-version v1.26.2 \
--target-namespace default \
--control-plane-machine-count=3 \
--worker-machine-count=1 \
--flavor=powervs-cloud-provider | kubectl apply -f -

Here one may not know
What is IBMPOWERVS_IMAGE_NAME, IBMPOWERVS_NETWORK_NAME so on.

Anything else you would like to add:
[Miscellaneous information that will assist in solving the issue.]

[arshadd-b]

/assign