[MLIR][Linalg] Fixed obsolete examples in the MLIR Linalg Dialect doc

This diff fixes some obsolete examples in the Linalg dialect documentation: https://mlir.llvm.org/docs/Dialects/Linalg/

These examples are used to explain the basic properties of the Linalg dialect, which are not automatically generated from TableGen and are using out-of-date MLIR/Linalg syntax.

This diff extends each example by adding essential attributes and changing its syntax to make it processible by `mlir-opt`. There is also a command attached to each example that says how the example can be processed.

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D84229