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>
'make htmldocs' does not use file doc/board/st/st.rst because the name
matches the directory name. Let's rename it to st-dt.rst.
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>