docs: expand S3-compatible servers section

[davidmyriel]

The S3-compatible section shows endpointurl but never credentials, which is
where first-time users get stuck (e.g. discuss.dvc.org/t/777, #1280). This
adds:

• a credentials example (same --local / env-var methods as Amazon S3)
• an endpoint URL table for common services (Ceph, R2, DigitalOcean, IBM COS,
  MinIO, Nebius, Tigris), formats verified against each provider's docs
• two troubleshooting notes in the existing admonition: SignatureDoesNotMatch
  from signing-region validation, and ssl_verify for self-signed certs

I work at Tigris — it's one row in the table; the rest is vendor-neutral.

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