net: Move XFRM documentation into its own subdirectory

XFRM docs are currently reside in Documentation/networking directory,
yet these are distinctive as a group of their own. Move them into xfrm
subdirectory.

Reviewed-by: Randy Dunlap <[e-mail]>
Tested-by: Randy Dunlap <[e-mail]>
Signed-off-by: Bagas Sanjaya <[e-mail]>
Signed-off-by: Steffen Klassert <[e-mail]>

Documentation/networking/index.rst

···
    vxlan
    x25
    x25-iface
-   xfrm_device
-   xfrm_proc
-   xfrm_sync
-   xfrm_sysctl
+   xfrm/index
    xdp-rx-metadata
    xsk-tx-metadata
 

Documentation/networking/xfrm/index.rst

···
+.. SPDX-License-Identifier: GPL-2.0
+
+==============
+XFRM Framework
+==============
+
+.. toctree::
+   :maxdepth: 2
+
+   xfrm_device
+   xfrm_proc
+   xfrm_sync
+   xfrm_sysctl

Documentation/networking/xfrm_sync.rst Documentation/networking/xfrm/xfrm_sync.rst

···
 .. SPDX-License-Identifier: GPL-2.0
 
-====
-XFRM
-====
+=========
+XFRM sync
+=========
 
 The sync patches work is based on initial patches from
 Krisztian <[e-mail]> and others and additional patches