Update human demonstration dataset guidance

[Battleplus]

What this does

• Clarifies that sample demonstrations are no longer bundled with robosuite.
• Points users to the current robomimic dataset download and format-conversion documentation.
• Distinguishes robosuite-collected demo.hdf5 playback from robomimic dataset processing.
• Replaces the removed sample-data path in the playback script's example and CLI help.

Addresses the documentation mismatch reported in #637.

Validation

• uvx --from pre-commit pre-commit run --files docs/algorithms/demonstrations.md robosuite/scripts/playback_demonstrations_from_hdf5.py
• python -m compileall -q robosuite/scripts/playback_demonstrations_from_hdf5.py
• git diff --check
• Verified all newly linked robomimic documentation pages return HTTP 200.