docs: sphinxify ubsan.txt and move it to dev-tools

Acked-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
diff --git a/Documentation/dev-tools/tools.rst b/Documentation/dev-tools/tools.rst
index 0500e65..2d11297 100644
--- a/Documentation/dev-tools/tools.rst
+++ b/Documentation/dev-tools/tools.rst
@@ -19,3 +19,4 @@
    kcov
    gcov
    kasan
+   ubsan