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

[abdulawalarif]

serverpod/serverpod#3470 is a pub.dev packaging audit. This PR updates the File uploads page so it is the canonical setup path those packages can link to. It does not close that issue.

S3, GCP, and R2 each get one default setup path. Native GCP and custom S3-compatible endpoints stay as labelled variants. The GCP HMAC region example is us-central1 instead of auto.

Rebased onto main after #765. The Upload a file and Access stored files sections keep the renamed storage API. Unsupported options, including preventOverwrite on providers that do not support it, throw.

Test plan

• Open Next → Concepts → Endpoints & APIs → File uploads.
• Confirm Upload a file and Access stored files still use the renamed storage API from docs: Document updated cloud storage interface #765.
• Walk HMAC GCP, native GCP, S3, and R2: install, import, constructor, and passwords or env vars.
• Confirm the custom S3-compatible section is last and is not the default path.
• Check links to Configuration, Sessions, pub.dev packages, GCP HMAC keys, and Cloudflare R2 tokens.

[developerjamiu]

Closes serverpod/serverpod#3470 points at "Improve pub.dev readiness for all Serverpod packages", an open framework issue about package READMEs and pub scores. Cross-repo closing keywords fire, so merging this would close it. Remove the line.

[abdulawalarif]

Thanks for the review:
I addressed the suggestions.

[developerjamiu]

The storage API was renamed in serverpod/serverpod#5621 yesterday, and #765 updated this page on main this morning. That's why the branch conflicts now.

The overlap is the first two sections, "Upload a file" and "Access stored files". Main has rewritten both for the new API, so your edits there aren't needed any more.

The rest of your work still applies: the provider sections, the passwords.yaml examples, the pub.dev links, and the us-central1 fix. Rebase on main and keep those.

Line 33 also needs dropping. It says S3 and HMAC GCP ignore preventOverwrite, which was correct when you wrote it. It now throws instead, and main has the new wording.

[abdulawalarif]

The branch is rebased, and the old preventOverwrite ignore wording is gone