Add documentation for UCloud US3 using S3-compatible storage policy

[summerboy2134]

Hi Cloudreve maintainers,

UCloud US3 is an object storage service provided by UCloud. US3 provides S3-compatible APIs, so Cloudreve users can configure it through the existing S3 Compatible storage policy.

We would like to contribute documentation for configuring UCloud US3 in Cloudreve, similar to the existing S3-compatible provider docs such as Backblaze B2, Cloudflare R2, Google Cloud Storage, and MinIO.

The proposed documentation will cover:

• creating a US3 bucket and access credentials
• filling in Cloudreve's S3 Compatible storage policy fields
• recommended Endpoint, Region, and path-style settings
• CORS configuration required for browser uploads
• common troubleshooting notes

This change would only affect the documentation repository and would not introduce new backend or frontend code.

Would this documentation contribution be acceptable?

Thanks!