docs(archival): document s3store endpoint options

[goanpeca]

What changed?

Added a short "S3-compatible endpoints" subsection to common/archiver/s3store/README.md documenting the endpoint and s3ForcePathStyle options of the s3store archival provider, with a small config snippet and a pointer to config/development-cass-s3.yaml. Docs only, no code changes.

Why?

Both options already exist and are already honored: common/config/config.go declares them on S3Archiver (endpoint, s3ForcePathStyle), and both history_archiver.go and visibility_archiver.go pass them straight through to s3.Options.BaseEndpoint and s3.Options.UsePathStyle. config/development-cass-s3.yaml depends on them to talk to localstack. The README configuration sample never mentions either key, so anyone pointing archival at an S3 API other than the AWS default has to read the source or reverse-engineer the dev config to discover they are supported.

How did you test it?

• covered by existing tests

Markdown-only change. I did verify that the snippet I documented unmarshals into config.S3Archiver exactly as written, so the key names and casing in the README match the yaml tags in common/config/config.go.

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