docs: fix cross-links after the refresh; remove the stale benchmark script

- docs/README.md links the improvement logs and June plans where the
  refresh archived them (docs/archive/).
- clawhdf5-py README: 'r+' creates and replaces attributes (compact or
  dense); only deleting them is unsupported.
- scripts/run-benchmarks.sh benchmarked the pre-rename rustyhdf5-format
  and overwrote BENCHMARKS.md; nothing referenced it. Removed.
- Cargo.toml descriptions no longer name rustyhdf5/edgehdf5; clawhdf5-gpu
  says it is not HDF5 I/O.
- benchmarks/cross_platform.sh pointed at a ROADMAP section that no longer
  exists.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

crates/clawhdf5-py/README.md

@@ -129,7 +129,8 @@ with clawhdf5.File("data.h5", "r+") as f:
   `str` is stored as a fixed-length UTF-8 string (h5py stores a
   variable-length one), so h5py reads it back as `bytes`.
 - Not supported (`NotImplementedError`, nothing written): creating or
-  deleting datasets, groups and attributes, writing compound fields by
+  deleting datasets and groups, deleting attributes (creating and
+  replacing them works, compact or dense), writing compound fields by
   name, variable-length data, HDF5 array types, and whatever
   `FileEditor` refuses (listed in `docs/known-issues.md`).