[SYCL][DOC] Make some refactoring for group sorting spec (#4666)

No changes for interfaces. Just some clarifications to avoid misunderstanding.

Following was done:
1. Moved sorting functions before sorters.
2. Added a comment for bubble_sort that this is not a recommended implementation for performance.
3. Added a comment that users shouldn’t create their own Group Helper. `group_with_scratchpad` is enough.
4. Changed the status for the Spec in the `README.md`.
5. Made an entire extension experimental.

Signed-off-by: Fedorov, Andrey <[email protected]>

Author: andreyfe1