feat(benchmarks): add matrix runner and leaderboard

spaces/benchmark-leaderboard/README.md

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+---
+title: LeRobot Benchmark Leaderboard
+emoji: 🤖
+colorFrom: yellow
+colorTo: orange
+sdk: gradio
+sdk_version: 5.29.0
+app_file: app.py
+pinned: false
+license: apache-2.0
+short_description: Benchmark history for LeRobot policy x benchmark runs
+---
+
+# LeRobot Benchmark Leaderboard
+
+This Space reads immutable benchmark rows from a Hugging Face dataset and shows:
+
+- Latest result per policy and benchmark
+- Historical trends over time
+- Direct links to uploaded eval and config artifacts
+
+## Configuration
+
+Set `BENCHMARK_RESULTS_REPO` in the Space settings if you want to point the UI
+at a different public dataset. The default is:
+
+- `lerobot/benchmark-history`