Add AbstractJsonExtractorOutputGuardrail to guardrails docs

[mariofusco]

/cc @cescoffier @geoand

[quarkus-bot[bot]]

Status for workflow Build (on pull request)

This is the status report for running Build (on pull request) on commit 9f6b857ac5d15f89cf5d1af27df3142921c78bf8.

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It should be safe to merge provided you have a look at the other checks in the summary.