📝 Document GIF-to-PNG conversion and current source import

[AnandChowdhary]

Summary

• Explain that qrcode produces GIF data URLs and show a tested, optional ImageScript conversion to PNG for 默认返回gif格式的base64，有没有参数指定或修改返回的格式？ #8 (no new library dependency).
• Point the getting-started and conversion snippets at a pinned current source revision because the deno.land/x tag remains v2.0.0 with outdated return types (Publish up-to-date code on https://deno.land/x/qrcode #7). This does not publish a release or close either issue.

Validation

• deno test (7 passed)
• deno check /tmp/oss-qrcode-convert.ts and deno run --allow-write --allow-net=jsr.io /tmp/oss-qrcode-convert.ts: produced a 249×249 PNG
• deno lint and deno fmt --check on the extracted conversion snippet
• git diff --check and added-line security scan