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/SConscript b/SConscript
index e6ef73c..7ea59d2 100644
--- a/SConscript
+++ b/SConscript
@@ -511,16 +511,15 @@
 # Common backend files
 lib_files = filelist['common']
 
+# Fixed format GEMM kernels.
+if env['fixed_format_kernels']:
+    arm_compute_env.Append(CPPDEFINES = ['ARM_COMPUTE_ENABLE_FIXED_FORMAT_KERNELS'])
+
 # Experimental files
 # Dynamic fusion
 if env['experimental_dynamic_fusion']:
     lib_files += filelist['experimental']['dynamic_fusion']
 
-# Fixed format GEMM kernels.
-if env['experimental_fixed_format_kernels']:
-    arm_compute_env.Append(CPPDEFINES = ['ARM_COMPUTE_ENABLE_FIXED_FORMAT_KERNELS'])
-
-
 # Logging files
 if env["logging"]:
     lib_files += filelist['logging']
@@ -596,8 +595,9 @@
     else:
         attrs = get_attrs_list(env, env['data_type_support'], env['data_layout_support'])
 
-    if env['experimental_fixed_format_kernels']:
-        attrs.append("experimental_fixed_format_kernels")
+
+    if env['fixed_format_kernels']:
+        attrs.append("fixed_format_kernels")
 
     # Setup data-type and data-layout files to include
     cpu_operators = custom_operators if use_custom_ops else filelist['cpu']['operators'].keys()