Update documentation on micro kernels

Refer to main matmul folder for supported data types
and available micro kernels.

Remove reference to micro kernels and winograd transform as
winograd micro kernels are not available now.

Signed-off-by: Felix Thomasmathibalan <felixjohnny.thomasmathibalan@arm.com>

Approved-by: Jakub Sujak <jakub.sujak@arm.com>