Use master for links to docs in source (#10866)

Resolved Conflicts:
        .github/PULL_REQUEST_TEMPLATE.md
        rllib/agents/dqn/apex.py
        rllib/agents/dqn/dqn.py

python/ray/setup-dev.py

@@ -63,7 +63,7 @@ if __name__ == "__main__":
     print("Created links.\n\nIf you run into issues initializing Ray, please "
           "ensure that your local repo and the installed Ray are in sync "
           "(pip install -U the latest wheels at "
-          "https://docs.ray.io/en/latest/installation.html, "
+          "https://docs.ray.io/en/master/installation.html, "
           "and ensure you are up-to-date on the master branch on git).\n\n"
           "Note that you may need to delete the package symlinks when pip "
           "installing new Ray versions to prevent pip from overwriting files "

python/ray/tests/test_metrics.py

@@ -402,7 +402,7 @@ def test_memory_dashboard(shutdown_only):
     """Test Memory table.
 
     These tests verify examples in this document.
-    https://docs.ray.io/en/latest/memory-management.html#debugging-using-ray-memory
+    https://docs.ray.io/en/master/memory-management.html#debugging-using-ray-memory
     """
     addresses = ray.init(num_cpus=2)
     webui_url = addresses["webui_url"].replace("127.0.0.1", "http://127.0.0.1")