Document DE/TMM UTM URL tracking campaigns in Developer Evangelism handbook

Why is this change being made?

Developer Evangelism and Technical Marketing uses UTM URL tracking in the Community App Campaign Manager. We have experimented with different campaigns, and UTM codes which are now used in production.

This MR documents the common campaigns we use, and should be helpful as orientation for @gitlab-de on where to create new shortened URLs, and different social media share target URLs.

Author Checklist

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