Move Perf documentation to the testing section

BUG=skia:
NOTRY=true
DOCS_PREVIEW= https://skia.org/?cl=1280533003

Review URL: https://codereview.chromium.org/1280533003

site/dev/tools/index.md

@@ -1,6 +1,5 @@
 Tools
 =====
 
-Developer tools for accessing the health of Skia, such as
-`nanobench`, `dm`, and [Skia Perf](https://perf.skia.org).
+Developer tools for working in Skia.