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/develop/driver-model/migration.rst b/doc/develop/driver-model/migration.rst
index fe1ae21..03fea94 100644
--- a/doc/develop/driver-model/migration.rst
+++ b/doc/develop/driver-model/migration.rst
@@ -100,7 +100,7 @@
 other base driver model options in time for inclusion in the 2021.10 release.
 
 CFG_SYS_TIMER_RATE and CFG_SYS_TIMER_COUNTER
---------------------------------------------------
+--------------------------------------------
 Deadline: 2023.01
 
 These are legacy options which have been replaced by driver model.