Add documentation for UCloud US3 using S3-compatible storage policy

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!

Start by reading the existing S3-compatible provider documentation for Backblaze B2, Cloudflare R2, Google Cloud Storage, and MinIO, then locate the corresponding documentation area in this repository. Add a UCloud US3 guide covering bucket and credentials setup, policy fields, endpoint, region, path-style settings, CORS, and troubleshooting; done means the configuration steps are complete and consistent with the existing provider docs.

Written by the indexing model from the issue text.