Add Daml Script reference to generated-docs update automation

[danielporterda]

Context

PR #1105 adds Daml Script as a generated reference surface, including its source configuration, generator, generated pages, navigation, and aggregate generate:all-reference-docs integration.

The scheduled generated-docs workflow runs scripts/update_generated_reference_prs.py --targets all, but Daml Script is not registered in either:

• scripts/update_generated_reference_sources.py (ALL_SOURCES)
• scripts/update_generated_reference_prs.py (UPDATE_TARGETS)

Problem

After PR #1105 merges, the Daml Script source pin and checked-in generated pages will not be updated by the daily/manual generated-reference automation. They can silently become stale while the other generated reference surfaces continue to receive update PRs.

Acceptance criteria

• Add a Daml Script source updater that resolves and records the latest supported Daml Script SDK artifact/version.
• Register Daml Script in ALL_SOURCES so update:generated-reference-sources -- --source daml-script and the default/all path include it.
• Register a Daml Script target in UPDATE_TARGETS, including its source config and generated output paths.
• Ensure the scheduled --targets all workflow can create or update a focused generated-reference PR when the Daml Script pin changes.
• Add focused tests covering target selection, source-pin updates, allowed changed paths, and the no-change/idempotent case.

Follow-up from #1105.