Added simple Ape-X baseline for documentation

baselines/apex_tree_obs/apex.yaml

+flatland-sparse-small-tree-fc-apex:
+    run: APEX
+    env: flatland_sparse
+    stop:
+        timesteps_total: 5000000 # 5e6
+    checkpoint_freq: 10
+    checkpoint_at_end: True
+    keep_checkpoints_num: 5
+    checkpoint_score_attr: episode_reward_mean
+    config:
+        num_workers: 3
+        num_envs_per_worker: 5
+        num_gpus: 0
+
+        env_config:
+            observation: tree
+            observation_config:
+                max_depth: 2
+                shortest_path_max_depth: 30
+
+            generator: sparse_rail_generator
+            generator_config: small_v0
+
+            wandb:
+                project: flatland
+                entity: masterscrat
+                tags: ["small_v0", "tree_obs", "apex"]
+
+        model:
+            fcnet_activation: relu
+            fcnet_hiddens: [256, 256]
+            vf_share_layers: True