[SYCL][Doc] Move free function kernel extension to experimental (#20932)

Modulo any outstanding bugs, the free function kernel extension has now
been fully implemented.
This PR moves the extension to the experimental folder.

Author: lbushi25

sycl/doc/design/CommandGraph.md

@@ -255,7 +255,7 @@ collisions. In practice there are only a few current scenarios where `set_arg()`
 can be used:
 
 * The proposed ["Free Function Kernel"
-  extension](../extensions/proposed/sycl_ext_oneapi_free_function_kernels.asciidoc)
+  extension](../extensions/experimental/sycl_ext_oneapi_free_function_kernels.asciidoc)
 * OpenCL interop kernels created from SPIR-V source at runtime.
 
 A workaround for the lambda capture issues is the "Whole-Graph Update" feature.

…xt_oneapi_free_function_kernels.asciidoc …xt_oneapi_free_function_kernels.asciidocsycl/doc/extensions/proposed/sycl_ext_oneapi_free_function_kernels.asciidoc renamed to sycl/doc/extensions/experimental/sycl_ext_oneapi_free_function_kernels.asciidoc sycl/doc/extensions/proposed/sycl_ext_oneapi_free_function_kernels.asciidoc renamed to sycl/doc/extensions/experimental/sycl_ext_oneapi_free_function_kernels.asciidoc

@@ -55,16 +55,13 @@ This extension also depends on the following other SYCL extensions:
 
 == Status
 
-This is a proposed extension specification, intended to gather community
-feedback.
-Interfaces defined in this specification may not be implemented yet or may be
-in a preliminary state.
-The specification itself may also change in incompatible ways before it is
-finalized.
+This is an experimental extension specification, intended to provide early
+access to features and gather community feedback.  Interfaces defined in this
+specification are implemented in {dpcpp}, but they are not finalized and may
+change incompatibly in future versions of {dpcpp} without prior notice.
 *Shipping software products should not rely on APIs defined in this
 specification.*
 
-
 == Overview
 
 This extension introduces a new way to define a kernel as a simple C++