docs: add the image.upscale job type reference

[a-essawy]

Adds jobs/image-upscale.mdx and its nav entry for the image.upscale job type, which goes live alongside the monorepo PR.

Covers sizing (factor and target height), the five params, transparency, the png output contract, and the output-size limit with the real refusal message.

Notes

• SDK-first in every CodeGroup, per house convention.
• No em-dashes or semicolons in prose (writing-style.md).
• The size-limit section documents a VALIDATION_ERROR, not a generation failure: an oversized request is refused before the model loads, so it is billed as a few seconds of container time rather than a full render.

Test plan

• Mintlify preview renders the page and the nav entry
• Code samples match the live staging API (verified against api-staging.rendobar.com)

Do not merge before the monorepo PR deploys, or the page documents a job type that is not yet live.