[Synthetics-docs] Update docs to include JSON export reference

[marcadev-fc]

What does this PR do? What is the motivation?

This update references the ability to export result JSONs directly from the UI.
Users can now reference specific values from these JSONs in their monitor notifications.

Merge instructions

Merge readiness:

• Ready for merge

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Additional notes

[github-actions]

Preview links (active after the build_preview check completes)

Modified Files

• https://docs-staging.datadoghq.com/adam.ghaoul/update_template_variables_docs/synthetics/notifications/template_variables

[maycmlee]

Just a small edit