WIP 2.19 Porting Guide stub with data tagging

[nitzmahone]

We're linking to this PR as temporary documentation for the pending Data Tagging changes to core.

This PR won't include any other 2.19-specific Porting Guide entries. I assume once the feature has merged to devel, we'll merge this and create new PRs for other entries, or split an empty stub into a separate PR and merge it earlier if folks are champing at the bit to write non-DT Porting Guide entries.

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