Documentation: Add usecases, design and interface for core scheduling

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