fix: derive backend support matrix from repo facts

[TATP-233]

Summary

• replace the hand-maintained backend support table with a generated matrix derived from registry facts, owner YAMLs, and test coverage
• add a reusable support-matrix generator plus a thin refresh script for docs
• add docs guardrails and regression tests so the matrix cannot drift silently again

Linked Work

• Issue: Fixes Work: 从 registry / config / tests 生成支持矩阵 #164
• Milestone: none

Validation

• make check
• uv run pytest -m "not slow"
• Additional task-specific validation listed below

Commands actually run:

UV_CACHE_DIR=.uv-cache uv run python scripts/generate_support_matrix.py --write
UV_CACHE_DIR=.uv-cache uv run pytest tests/scripts/test_support_matrix.py tests/scripts/test_check_docs.py -q
UV_CACHE_DIR=.uv-cache make check
UV_CACHE_DIR=.uv-cache make test
UV_CACHE_DIR=.uv-cache uv run pytest -m "not slow and not veryslow" --ignore=tests/algos/test_mlx_ppo.py

make test aborts during collection because tests/algos/test_mlx_ppo.py triggers a host-level mlx.core import abort instead of reaching pytest.importorskip(...).
The broader non-slow run excluding that file still hits pre-existing IPC failures and later another train_mlx_ppo.py import abort inside tests/scripts/test_train_scripts.py.
The support-matrix-specific tests added in this PR pass.

Impact

• Backend impact: both
• Platform impact: both
• Training effect expected: no

Artifacts

• W&B: none
• benchmark result: none
• video / screenshot: none
• ONNX / checkpoint: none

Checklist

• Added or updated tests where needed
• Updated docs if behavior or workflow changed
• Linked the driving issue
• Noted any follow-up work explicitly