chore: trim reference/ to docs only (5.8MB -> 1.0MB)

- thinktrader_docs: keep .txt (extracted text), drop .html duplicates (36->18 files)
- xtquant_big_convert: keep README/LICENSE/CHANGELOG/.gitignore + docs/, drop src/tests/examples/benches/qmt-trader (project code, not referenced by bridge)
- un-ignore reference/xtquant_big_convert/.gitignore (now a normal tracked file)

reference/xtquant_big_convert/.gitignore

@@ -0,0 +1,28 @@
+__pycache__/
+*.py[cod]
+*.pyo
+*.pyd
+.Python
+.venv/
+venv/
+env/
+.pytest_cache/
+.mypy_cache/
+.ruff_cache/
+build/
+dist/
+*.egg-info/
+
+# Local runtime secrets. Keep real account ids, Redis passwords and QMT paths out of git.
+# bigqmt_signal_trader_local_config.py
+# src/bigqmt_signal_trader_local_config.py
+# bigqmt_signal_trader_client_config.py
+# src/bigqmt_signal_trader_client_config.py
+*.local.py
+*.log
+*.pid
+
+# Generated evidence from standalone/QMT backtest bridge runs.
+backtest_runs/
+qmt_backtest_runs/
+.workbuddy/