docs: Make file-upload storage setup the default path for S3, GCP, and R2

[abdulawalarif]

Summary

• Reworks the file-uploads concept page so each cloud provider has one default setup path (HMAC GCP, AWS S3, Cloudflare R2), with native GCP and custom S3-compatible endpoints as labelled variants.
• Documents serverpod_cloud_storage_s3_compat as a building block for services such as LocalStack, not as the package most readers should add by hand.
• Corrects the GCP HMAC region example (us-central1 instead of auto) and states which providers actually enforce preventOverwrite.
• Adds pub.dev package links, password-file examples with env-var names from the packages, and a Related section to configuration and sessions.

Closesserverpod/serverpod#3470.

Test plan

• Open Next → Concepts → Endpoints & APIs → File uploads.
• Confirm the upload flow still covers description, verify, and client upload, with path rules in the main body (no info callout).
• Walk the GCP HMAC, native GCP, S3, and R2 sections and check that install, import, constructor, and passwords.yaml / env vars match.
• Confirm the custom S3-compatible section is at the bottom and does not read as the default path.
• Check in-page links: Configuration, Sessions, pub.dev packages, GCP HMAC docs, Cloudflare R2 tokens.