docs: document storageContainerName field for explicit CSC selection

## Summary

KServe v0.20 added a `storageContainerName` field to InferenceService for explicitly selecting which ClusterStorageContainer (CSC) to use for model download. No documentation exists for this feature.

## What needs to be documented

- The `storageContainerName` field on InferenceService spec
- When to use it: environments with multiple ClusterStorageContainers where automatic matching may be ambiguous
- How it differs from the default URI-pattern-based CSC matching
- Example YAML showing explicit CSC selection
- Update the storage containers documentation (`docs/model-serving/storage/storage-containers.md`)

## Related PR

- https://github.com/kserve/kserve/pull/5314

## Since

KServe v0.20

Start with docs/model-serving/storage/storage-containers.md and review the related PR #5314 for the KServe v0.20 behavior. Document the storageContainerName field, explain when explicit selection is needed versus URI-pattern matching, and add an example YAML configuration; the storage containers documentation should cover all requested guidance.

Written by the indexing model from the issue text.