Documentation: Add usecases, design and interface for core scheduling

mainline inclusion
from mainline-v5.14-rc1
commit 0159bb02
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I5OOWG
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=0159bb020ca9a43b17aa9149f1199643c1d49426



--------------------------------------------------------------------------

Now that core scheduling is merged, update the documentation.

Co-developed-by: Chris Hyser <chris.hyser@oracle.com>
Signed-off-by: Chris Hyser <chris.hyser@oracle.com>
Co-developed-by: Josh Don <joshdon@google.com>
Signed-off-by: Josh Don <joshdon@google.com>
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20210603013136.370918-1-joel@joelfernandes.org


Signed-off-by: Lin Shengwang <linshengwang1@huawei.com>
Reviewed-by: lihua <hucool.lihua@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>