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/SConstruct b/SConstruct
index 2e20931..d6f15c0 100644
--- a/SConstruct
+++ b/SConstruct
@@ -126,7 +126,7 @@
             ├── fixtures
             └── Neon\n""", "", PathVariable.PathAccept),
     BoolVariable("experimental_dynamic_fusion", "Build the experimental dynamic fusion files", False),
-    BoolVariable("experimental_fixed_format_kernels", "Enable fixed format kernels for GEMM", False),
+    BoolVariable("fixed_format_kernels", "Enable fixed format kernels for GEMM", False),
     BoolVariable("mapfile", "Generate a map file", False),
     ListVariable("custom_options", "Custom options that can be used to turn on/off features", "none", ["disable_mmla_fp"]),
     ListVariable("data_type_support", "Enable a list of data types to support", "all", ["qasymm8", "qasymm8_signed", "qsymm16", "fp16", "fp32", "integer"]),