docs: add multipart binary usage examples

[adamcavendish]

Summary

• Add a mdBook page explaining multipart upload wrappers, filename fallback behavior, and raw binary body handling.
• Add a reusable multipart plus octet-stream OpenAPI example under examples/multipart-binary.
• Correct generator CLI examples to use repeatable --generators flags and separate output directories per target.

Validation

• mdbook build docs
• cargo run --quiet -- generate --input examples/multipart-binary/openapi.yaml --output /tmp/openapi-nexus-multipart-example-ts --generators typescript-fetch
• cargo fmt --all -- --check