Document how to make a cluster a spoke of multiple hubs

[rolandkool]

Describe the enhancement

OCM is quite flexible. It is possible for a cluster to be both a Hub and a Spoke at the same time. It is also possible for a cluster to be a Spoke of multiple Hubs. However, how to set this up is not documented. It would be helpful to do so.

Some steps I had to take to make it work are:

• create a unique new namespace for second hub called open-cluster-management-agent-<newcluster>
• manually create the bootstrap-hub-config secret. I used another one as basis and made the necessary adjustments like updating the token. This step in particular requires some explanation.
• create a klusterlet resource with the name of and pointed it to the new namespace

[zhujian7]

/cc @elgnay

[qiujian16]

/assign @elgnay

[github-actions]

This issue is stale because it has been open for 120 days with no activity. After 14 days of inactivity, it will be closed. Remove the stable label to prevent this issue from being closed.

[elgnay]

Created two solutions and a scenario for this issue:

• 📖 add solutions of multiple hubs #304
• add a scenario of multiple hub open-cluster-management-io.github.io#392

[elgnay]

Closing the issue for the proposed doc has been merged.