Add MinIO round-trip markdown demo

[perryrighthere]

Motivation

MinerU already provides local CLI, API, and HTTP client examples, but it does not include an official example for an object-storage-first workflow.

This PR adds a MinIO round-trip demo that shows how to upload a local file to MinIO, read it back from MinIO for parsing, upload the generated artifacts back to MinIO, and produce Markdown whose image references point to MinIO object URLs.

Modification

• add demo/minio_markdown_demo.py as an official MinIO round-trip parsing example
• support MinIO endpoint, access key, and secret key configuration via CLI arguments, environment variables, and mineru.json fallback
• rewrite generated Markdown and JSON image references to MinIO object URLs
• save the downloaded Markdown locally after the MinIO round-trip completes
• document the new example in docs/zh/usage/quick_usage.md and docs/en/usage/quick_usage.md
• add lightweight unit tests for helper behavior in tests/unittest/test_minio_markdown_demo.py

BC-breaking

None.

Use cases

• integrate MinerU into MinIO or S3-compatible object storage workflows
• provide an official example for storage-backed parsing pipelines
• generate Markdown that can directly reference MinIO-hosted images in downstream systems

Validation

• python -m py_compile demo/minio_markdown_demo.py
• .venv/bin/python -m unittest discover -s tests/unittest -p 'test_minio_markdown_demo.py'

[github-actions]

All contributors have signed the CLA ✍️ ✅
_{Posted by the CLA Assistant Lite bot.}

[perryrighthere]

I have read the CLA Document and I hereby sign the CLA