Add sched_ext concept page

[dylandreimerink]

We now have pages for sched_ext kfuncs, the struct_ops and its helper library. The only thing that remains is to write a concept page that explain what sched_ext is and how it operates.

Important points to cover:

• How tasks move from the existing scheduler to SCX
• SCX core and BPF scheduler divide
• How does SCX core (roughly) call into the BPF scheduler
• What is a runqueue
• What is a DSQ
  • FIFO DSQ vs vtime DSQ
  • Local vs Global vs User DSQs
• CPU masks
• Task weight
• cGroup scheduling