Add Step to "Install from Source" on Rocky Linux

[JoshInnis]

When following the documentation, I had to source the greenplum_path.sh before I was able to run the create-demo-cluster step.

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Change logs

While installing Cloudberry from source (on Rocky 9) I had to perform 1 more step to get the example working. To note, other than this step the instructions for Rocky 8 and RHEL 8 work with Rocky 9 and perhaps the tab names should be updated as well.

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