docs: perf: move to the admin-guide

The perf infrastructure is used for userspace to track issues.
At least a good part of what's described here is related to
it.

So, add it to the admin-guide.

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

Documentation/admin-guide/index.rst

@@ -38,6 +38,7 @@ problems and bugs in particular.
    ramoops
    dynamic-debug-howto
    init
+   perf/index
 
 This is the beginning of a section with information of interest to
 application developers.  Documents covering various aspects of the kernel

Documentation/admin-guide/perf/index.rst

@@ -1,5 +1,3 @@
-:orphan:
-
 ===========================
 Performance monitor support
 ===========================

MAINTAINERS

@@ -1155,7 +1155,7 @@ APPLIED MICRO (APM) X-GENE SOC PMU
 M:	Khuong Dinh <khuong@os.amperecomputing.com>
 S:	Supported
 F:	drivers/perf/xgene_pmu.c
-F:	Documentation/perf/xgene-pmu.rst
+F:	Documentation/admin-guide/perf/xgene-pmu.rst
 F:	Documentation/devicetree/bindings/perf/apm-xgene-pmu.txt
 
 APTINA CAMERA SENSOR PLL
@@ -7262,7 +7262,7 @@ M:	Shaokun Zhang <zhangshaokun@hisilicon.com>
 W:	http://www.hisilicon.com
 S:	Supported
 F:	drivers/perf/hisilicon
-F:	Documentation/perf/hisi-pmu.rst
+F:	Documentation/admin-guide/perf/hisi-pmu.rst
 
 HISILICON ROCE DRIVER
 M:	Lijun Ou <oulijun@huawei.com>

drivers/perf/qcom_l3_pmu.c

@@ -8,7 +8,7 @@
  * the slices. User space needs to aggregate to individual counts to provide
  * a global picture.
  *
- * See Documentation/perf/qcom_l3_pmu.rst for more details.
+ * See Documentation/admin-guide/perf/qcom_l3_pmu.rst for more details.
  *
  * Copyright (c) 2015-2017, The Linux Foundation. All rights reserved.
  */