[MLIR][doc] Make summary appear different than discription

This patch aims to clarify which part of the docs is the summary and which
part is the description.

I did a preview of the output and think the suggested bold and italic style
is much clearer and looks reasonably nice.

I've considered also to enforce capitalization of the first letter of the
summary and ending with a dot (as discussed in
https://reviews.llvm.org/D151649), but it seems to me that the tradeoff in
extra test running time is not worth it. Currently, `emitOpDoc` is not
triggered for every `summary` during `check-mlir`.

Reviewed By: jpienaar

Differential Revision: https://reviews.llvm.org/D152621