docs: document model cache configuration

[Mirochill]

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• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Summary

• Document the current FastEmbed model cache default in README.md and docs/index.md.
• Show how to use a persistent cache with cache_dir.
• Show how to set FASTEMBED_CACHE_PATH for a default cache location, and mention mounting that directory for container restarts.

Helps #569.

Validation

• git diff --check
• git show --check --stat --oneline HEAD
• rg -n "Model cache|FASTEMBED_CACHE_PATH|cache_dir=|fastembed_cache" README.md docs/index.md fastembed/common/utils.py
• Tests not run locally; documentation-only change.