docs(document): document json-placeholders conversion option and default placeholder protection

[yoshiamakino]

Documents the JSON placeholder protection workstream now live on prod:

• json-placeholders conversion option in the input_conversion_options reference (openapi.yaml, regenerated openapi.json): protect (default) keeps identifier-shaped placeholders such as {stars} verbatim; translate translates them. ICU MessageFormat skeletons are always protected with only branch text translated. Moves the pptx-only restriction onto the suppress-image-types bullet, since conversion options now apply to json too.
• Changelog entry (September 30): default placeholder protection for json documents + the opt-out.
• Best-practices JSON section: placeholder behaviour, the translate opt-out, ICU skeleton guarantees, and the two known caveats (branch text translated in isolation; missing plural categories fall back to other).

Behaviour verified live on prod 2026-09-29/30: default arm {stars} intact 5/5, ICU skeletons 6/6 intact on prod and api-test.

[mintlify]

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Project	Status	Preview	Updated
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