docs: sphinixfy gcov.txt and move to dev-tools

No textual changes beyond formatting.

Cc: Peter Oberparleiter <oberpar@linux.vnet.ibm.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
diff --git a/Documentation/dev-tools/tools.rst b/Documentation/dev-tools/tools.rst
index 9dcd023..404d044 100644
--- a/Documentation/dev-tools/tools.rst
+++ b/Documentation/dev-tools/tools.rst
@@ -17,3 +17,4 @@
    coccinelle
    sparse
    kcov
+   gcov