Document how to fix PDF views with LFS and proxy_download disabled

What does this MR do and why?

When LFS storage is configured with object storage and proxy_download is set to false, users attempting to view PDF files uploaded to LFS may see:

An error occurred while loading the file. Please try again later.

This can happen because the object storage provider rejects the PDF.js frontend request to load the PDF file since the GitLab domain is not allowed due to Cross-Origin Resource Sharing (CORS) restrictions. This commit documents that it is necessary to configure S3 CORS headers to make this work.

Closes #248100 (closed)

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