Documentation: KUnit: add intro to the getting-started page

Describe the objective of the Getting Started page, which should be a
brief and beginner-friendly walkthrough for running and writing tests,
showing the reader where to find detailed instructions in other pages.

Signed-off-by: Tales Aparecida <tales.aparecida@gmail.com>
Reviewed-by: Maíra Canal <mairacanal@riseup.net>
Reviewed-by:Sadiya Kazi<Sadiaykazi@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

Documentation/dev-tools/kunit/start.rst

@@ -4,6 +4,10 @@
 Getting Started
 ===============
 
+This page contains an overview of the kunit_tool and KUnit framework,
+teaching how to run existing tests and then how to write a simple test case,
+and covers common problems users face when using KUnit for the first time.
+
 Installing Dependencies
 =======================
 KUnit has the same dependencies as the Linux kernel. As long as you can