s3: add RelAix.Storage provider - #9994

What does this change do?

Adds RelAix.Storage as an S3 provider. RelAix.Storage is S3 compatible object storage from RelAix Networks GmbH (Aachen, Germany), operated in its own data centres.

• backend/s3/provider/RelAix.yaml with the two locations s3-gw1.relaix.net (Aachen-1) and s3-gw2.relaix.net (Aachen-2). Both sign with the default region, so no region question is asked.
• Docs: provider section with an rclone config transcript in docs/content/s3.md, plus entries in README.md and docs/content/_index.md. The autogenerated parts of s3.md are untouched.

Linked issue

None – a new provider file plus docs, following backend/s3/README.md.

For new or changed backends

Integration tests against Aachen-1 (go test ./backend/s3 -remote RelAix:<bucket> -run TestIntegration -v): 186 passed, 18 skipped, 1 failed:

• TestIntegration/FsMkdir/FsPutFiles/Internal/ObjectLock/Multipart: the gateway ignores the Object Lock headers sent with CreateMultipartUpload, so the completed object has no lock mode. Single-part uploads with the same headers and bucket default retention (also for multipart uploads) work. This is a limitation of our gateway that we are working on, not of the provider config. The docs section tells users to set object_lock_set_after_upload = true when they lock objects on upload; verified against Aachen-1 with a 20 MB multipart upload (lock set, delete without bypass denied).

go test ./fs/operations and go test ./fs/sync with -remote RelAix:<bucket>: ok.

Test account for the integration test server: not set up yet – happy to discuss if you need one.

Checklist

• This change is trivial OR it has been discussed and agreed in the linked issue.
• I have read the contribution guidelines.
• (If I used AI tools to help write this code) I have read and understood the AI-assisted contributions guidance, and I have tested and take ownership of this change myself.
• I have added tests for all changes in this PR if appropriate.
• I have added documentation for the changes if appropriate.
• All commit messages are in house style.
• (Backend changes only) test_all passes for this backend and if submitting a new backend can provide a test account for the integration tester.
• This Pull Request is ready for review.