docs: classify the fragment's non-Lance referenced files by format

[LuciferYang]

Fragment::referenced_lance_files documents which of a fragment's referenced files it does not yield, and the list named two of the four kinds. A fragment can also reference external row-version metadata through created_at_version_meta and last_updated_at_version_meta, both RowDatasetVersionMeta::External(ExternalFile) just like the external row-id file.

Classify by format instead of enumerating, so the boundary stays correct as kinds are added: a deletion file is .arrow or .bin per DeletionFileType::suffix, and external row-id or row-version metadata is an ExternalFile, a (path, offset, size) byte range rather than a Lance file.

This matters because the omission reads as a statement about what is referenced at all. Dataset::referenced_files in #8097 refuses outright on each of those three external kinds, precisely because they are referenced but not enumerated by any per-file walk; a reader of this doc could reasonably conclude row-version metadata is not a referenced file.

Docs only, no code change. cargo fmt --all --check, cargo clippy -p lance-table --all-targets -- -D warnings, and RUSTDOCFLAGS="-D warnings" cargo doc -p lance-table --no-deps are clean; lance-table format tests pass (32).

[lance-gatekeeper]

✅ Gate recommendation: approve.

The latest merge from main leaves the documentation patch unchanged. The storage-based classification still matches the current deletion-file suffixes and external row-id and row-version byte-range representations without changing behavior.

[LuciferYang]

Thank you @Xuanwo