doc: shorten overlong title underlines

Title underlines should match the length of the title. Unfortunately
docutils only catches underlines that are too short.

Add some missing empty lines after titles.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
diff --git a/doc/board/thead/index.rst b/doc/board/thead/index.rst
index 41566d3..2c4b3fb 100644
--- a/doc/board/thead/index.rst
+++ b/doc/board/thead/index.rst
@@ -1,7 +1,7 @@
 .. SPDX-License-Identifier: GPL-2.0+
 
 T-HEAD
-========
+======
 
 .. toctree::
    :maxdepth: 1