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

Fold the sparse document into the development tools set; no changes to the
text itself beyond formatting.

Signed-off-by: Jonathan Corbet <corbet@lwn.net>
diff --git a/Documentation/dev-tools/tools.rst b/Documentation/dev-tools/tools.rst
index ae0c58c..d4bbda3 100644
--- a/Documentation/dev-tools/tools.rst
+++ b/Documentation/dev-tools/tools.rst
@@ -15,3 +15,4 @@
    :maxdepth: 2
 
    coccinelle
+   sparse