sched_ext: Documentation: scheduler: Document extensible scheduler class

sched_ext: Documentation: scheduler: Document extensible scheduler class

Add Documentation/scheduler/sched-ext.rst which gives a high-level overview
and pointers to the examples.

v6: - Add paragraph explaining debug dump.

v5: - Updated to reflect /sys/kernel interface change. Kconfig options
      added.

v4: - README improved, reformatted in markdown and renamed to README.md.

v3: - Added tools/sched_ext/README.

    - Dropped _example prefix from scheduler names.

v2: - Apply minor edits suggested by Bagas. Caveats section dropped as all
      of them are addressed.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: David Vernet <dvernet@meta.com>
Acked-by: Josh Don <joshdon@google.com>
Acked-by: Hao Luo <haoluo@google.com>
Acked-by: Barret Rhoden <brho@google.com>
Cc: Bagas Sanjaya <bagasdotme@gmail.com>