[mlir][scf] Add high-level documentation (#72313)

Credits for this description go to @ftynse in
<https://discourse.llvm.org/t/scf-dialect-vs-cfg/3524/2> and
@antiagainst in
<https://www.lei.chat/posts/mlir-codegen-dialects-for-machine-learning-compilers/>!
(Unless it's wrong of course, then it's my bad.)

EDIT: Also related:
<https://discourse.llvm.org/t/codegen-dialect-overview/2723

>.

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Co-authored-by: Mehdi Amini <joker.eph@gmail.com>