fix!: update eval-tasks -> benchmarks (#1032)

# What does this PR do?

- Update `/eval-tasks` to `/benchmarks`
- ⚠️ Remove differentiation between `app` v.s. `benchmark` eval task
config. Now we only have `BenchmarkConfig`. The overloaded `benchmark`
is confusing and do not add any value. Backward compatibility is being
kept as the "type" is not being used anywhere.

[//]: # (If resolving an issue, uncomment and update the line below)
[//]: # (Closes #[issue-number])

## Test Plan
- This change is backward compatible 
- Run notebook test with

```
pytest -v -s --nbval-lax ./docs/getting_started.ipynb
pytest -v -s --nbval-lax ./docs/notebooks/Llama_Stack_Benchmark_Evals.ipynb
```

<img width="846" alt="image"
src="https://github.com/user-attachments/assets/d2fc06a7-593a-444f-bc1f-10ab9b0c843d"
/>



[//]: # (## Documentation)
[//]: # (- [ ] Added a Changelog entry if the change is significant)

---------

Signed-off-by: Ihar Hrachyshka <ihar.hrachyshka@gmail.com>
Signed-off-by: Ben Browning <bbrownin@redhat.com>
Signed-off-by: Sébastien Han <seb@redhat.com>
Signed-off-by: reidliu <reid201711@gmail.com>
Co-authored-by: Ihar Hrachyshka <ihar.hrachyshka@gmail.com>
Co-authored-by: Ben Browning <ben324@gmail.com>
Co-authored-by: Sébastien Han <seb@redhat.com>
Co-authored-by: Reid <61492567+reidliu41@users.noreply.github.com>
Co-authored-by: reidliu <reid201711@gmail.com>
Co-authored-by: Yuan Tang <terrytangyuan@gmail.com>

docs/getting_started.ipynb

@@ -324,7 +324,7 @@
               "- vector_io\n",
               "container_image: null\n",
               "datasets: <span style=\"font-weight: bold\">[]</span>\n",
-              "eval_tasks: <span style=\"font-weight: bold\">[]</span>\n",
+              "benchmarks: <span style=\"font-weight: bold\">[]</span>\n",
               "image_name: together\n",
               "metadata_store:\n",
               "  db_path: <span style=\"color: #800080; text-decoration-color: #800080\">/Users/ashwin/.llama/distributions/together/</span><span style=\"color: #ff00ff; text-decoration-color: #ff00ff\">registry.db</span>\n",
@@ -508,7 +508,7 @@
               "- vector_io\n",
               "container_image: null\n",
               "datasets: \u001b[1m[\u001b[0m\u001b[1m]\u001b[0m\n",
-              "eval_tasks: \u001b[1m[\u001b[0m\u001b[1m]\u001b[0m\n",
+              "benchmarks: \u001b[1m[\u001b[0m\u001b[1m]\u001b[0m\n",
               "image_name: together\n",
               "metadata_store:\n",
               "  db_path: \u001b[35m/Users/ashwin/.llama/distributions/together/\u001b[0m\u001b[95mregistry.db\u001b[0m\n",