Update docs to account for clusterctl config -> generate renaming

[shysank]

/kind bug

What steps did you take and what happened:

Recently In capz, we found out that the docs are still pointing to old clusterctl config command which was renamed to clusterctl generate kubernetes-sigs/cluster-api-provider-azure#1476

What did you expect to happen:
Update docs to reflect the changes

Anything else you would like to add:
https://kubernetes.slack.com/archives/CEX9HENG7/p1624893595283300

Environment:

• Cluster-api-provider-aws version:
• Kubernetes version: (use kubectl version):
• OS (e.g. from /etc/os-release):

[randomvariable]

/kind documentation
/priority important-soon

[praveenghuge]

/assign