docs: sphinxify kcov.txt and move to dev-tools

Another document added to the dev-tools collection.

Cc: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
diff --git a/Documentation/dev-tools/tools.rst b/Documentation/dev-tools/tools.rst
index d4bbda3..9dcd023 100644
--- a/Documentation/dev-tools/tools.rst
+++ b/Documentation/dev-tools/tools.rst
@@ -16,3 +16,4 @@
 
    coccinelle
    sparse
+   kcov