Replace identifiers in Product Intelligence guide with link

Why is this change being made?

gitlab-org/gitlab#381574 (closed) added the table with all identifiers collected by Product Intelligence's Snowplow infrastructure to the Metrics Dictionary since the Metrics Dictionary is supposed to be the SSOT for collected data. We do not want to maintain this table in two places. This would make us run the risk of one of them getting outdated. As a result, we're replacing the table in the guide with a link to the Metrics Dictionary.

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