Update Data Sources (in handbook) with Redis explanation

Why is this change being made?

The change will support details about Redis source for data coming from www-gitlab-com. Initial implementation was under !5380

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Rob Parker Today at 5:23 PM @data-engineers It looks like we are now ingesting REDIS counter data from GitLab.com -- however I don't see this data > > source listed in our data source catalog. Where can I find more info about this new source?

Link to Slack conversation: https://gitlab.slack.com/archives/CSZMC7TJL/p1630506206019400

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