Move Perf documentation to the testing section

site/dev/tools/skiaperf.md

-Skia Perf
-=========
-
-[Skia Perf](https://perf.skia.org) is a web based interface for exploring
-performance data produced by `nanobench` and the code size bot. The data
-includes:
-
-  * The nanobench test times in ms.
-  * Total memory consumed during a nanobench run.
-  * Code size for various symbol types in bytes.
-
-All of the data can be plotted and also can be [analyzed using k-means
-clustering](https://perf.skia.org/clusters/).
-
-Calculations
-------------
-
-Skia Perf has the ability to perform calculations over the test data
-allowing you to build up interesting queries.
-
-For example, this query displays the [total code size of the library over time](https://perf.skia.org/#1877):
-
-    sum(fill(filter("config=memory&sub_result=bytes")))
-
-This query displays [the ratio of playback time in ms to the number of ops for desk\_wowwiki.skp](https://perf.skia.org/#1876):
-
-    ratio(
-      ave(fill(filter("name=desk_wowwiki.skp&sub_result=min_ms"))),
-      ave(fill(filter("name=desk_wowwiki.skp&sub_result=ops")))
-    )
-
-You can also use the data to answer questions like [how many tests were run per commit](https://perf.skia.org/#1878).
-
-    count(filter(""))
-
-See Skia Perf for the [full list of functions available](https://perf.skia.org/help).