Removes `experimental` from `experimental_fixed_format_kernels` flag

Renames `experimental_fixed_format_kernels` build option to
`fixed_format_kernels`.

Adds documentation for the flag covering basics:
- What fixed-format kernels are
- Why they're needed
- Which backend they're for (i.e. CPU)
- Some pointers on how to use them.

Resolves: ONCPUML-1253
Change-Id: I428c98614c309c9ffc32d0f32daa24740f7cb967
Signed-off-by: Nathan John Sircombe <nathan.sircombe@arm.com>
Reviewed-on: https://review.mlplatform.org/c/ml/ComputeLibrary/+/9523
Benchmark: Arm Jenkins <bsgcomp@arm.com>
Tested-by: Arm Jenkins <bsgcomp@arm.com>
Reviewed-by: SiCong Li <sicong.li@arm.com>
Comments-Addressed: Arm Jenkins <bsgcomp@arm.com>
diff --git a/filelist.json b/filelist.json
index efb0182..fc4aaeb 100644
--- a/filelist.json
+++ b/filelist.json
@@ -1649,7 +1649,7 @@
             "estate64": [
               "src/core/NEON/kernels/arm_gemm/kernels/a64_sgemv_pretransposed/generic.cpp"
             ],
-            "experimental_fixed_format_kernels": [
+            "fixed_format_kernels": [
               "src/core/NEON/kernels/arm_gemm/kernels/a64_ffhybrid_bf16fp32_mmla_6x16/generic.cpp",
               "src/core/NEON/kernels/arm_gemm/kernels/a64_ffhybrid_fp16_mla_6x32/generic.cpp",
               "src/core/NEON/kernels/arm_gemm/kernels/a64_ffhybrid_fp32_mla_6x16/generic.cpp",
@@ -1725,7 +1725,7 @@
               "src/core/NEON/kernels/arm_gemm/transform-sve.cpp",
               "src/core/NEON/kernels/arm_gemm/misc-sve.cpp"
             ],
-            "experimental_fixed_format_kernels": [
+            "fixed_format_kernels": [
               "src/core/NEON/kernels/arm_gemm/kernels/sve_ffhybrid_bf16fp32_mmla_6x4VL/generic.cpp",
               "src/core/NEON/kernels/arm_gemm/kernels/sve_ffhybrid_fp16_mla_6x4VL/a64fx.cpp",
               "src/core/NEON/kernels/arm_gemm/kernels/sve_ffhybrid_fp16_mla_6x4VL/generic.cpp",