docs: locking: add it to the main index

The locking directory is part of the Kernel API bookset. Add
it to the index file.
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

Documentation/index.rst

@@ -96,6 +96,7 @@ needed).
 
    driver-api/index
    core-api/index
+   locking/index
    accounting/index
    block/index
    cdrom/index

Documentation/locking/index.rst

-:orphan:
+.. SPDX-License-Identifier: GPL-2.0
 
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 locking