Update URLs in scheduler.md with correct folder path (#6198)

* Update URLs in scheduler.md with correct folder path

The `generic_scheduler.go` is moved under `scheduler` folder, not `scheduler/core` folder. This commit updated the description & the link to code file.
Note that `legacy_registry.go` is removed as well in master branch. We may need to fix the link and description as well after release.

* #6198 Updated scheduler part in contributors/devel/README.md, adding few doc links. Removed scheduler.md

* #6198 Removed 'Scheduler extensibility' part in scheduling_code_hierarchy_overview; removed scheduler framework plugins link in devel README

Author: jiekun

contributors/devel/README.md

@@ -117,10 +117,12 @@ See the [kubernetes/release](https://github.com/kubernetes/release) repository f
 
 ### SIG Scheduling
 
-* **Understanding the Kubernetes Scheduler** [scheduler.md](sig-scheduling/scheduler.md)
+* **Understanding the Kubernetes Scheduler** [scheduling_code_hierarchy_overview.md](sig-scheduling/scheduling_code_hierarchy_overview.md)
 
 * **Scheduler Algorithm in Kubernetes** [scheduler_algorithm.md](sig-scheduling/scheduler_algorithm.md)
 
+* **Understanding how Pods are queued in Kubernetes Scheduler** [scheduler_queues.md](sig-scheduling/scheduler_queues.md)
+
 * **Scheduler Benchmarking** [scheduler_benchmarking.md](sig-scheduling/scheduler_benchmarking.md)
 
 ### SIG Architecture