docs: namespace: move it to the admin-guide

As stated at the documentation, this is meant to be for
users to better understand namespaces.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

Documentation/admin-guide/index.rst

@@ -77,6 +77,7 @@ configure specific aspects of kernel behavior to your liking.
    thunderbolt
    LSM/index
    mm/index
+   namespaces/index
    perf-security
    acpi/index
 

Documentation/admin-guide/namespaces/index.rst

@@ -1,5 +1,3 @@
-:orphan:
-
 ==========
 Namespaces
 ==========