docs: add image byte-budget tutorial

[MarshallBear1]

Summary

Adds a written tutorial showing how to use React Native Skia's imperative API to decode a local image, resize it on an offscreen surface, and search across JPEG quality and dimensions until the decoded output fits a strict byte budget.

The tutorial includes:

• a complete reusable TypeScript implementation;
• exact Base64 decoded-byte accounting;
• explicit disposal of Skia data, image, paint, snapshot, and surface objects; and
• production notes on alpha, EXIF orientation, memory, request overhead, and server-side validation.

The example was extracted from production use, rewritten without application-specific dependencies or data, and checked against @shopify/react-native-skia 2.6.2. It is published under the MIT license.

Validation

• branch is based on the current upstream main commit
• tutorial and source resolve from the public default branch
• tutorial implementation passes a strict TypeScript no-emit check against React Native Skia 2.6.2
• known-value checks pass for Base64 byte accounting
• this PR changes only apps/docs/docs/tutorials.md
• git diff --check passes