doc: css: update code documentation directives style

New Sphinx version (or docutils) has slightly changed the output format
for code documentation directives. These changes try to mimic previous
behavior, even though it does not achieve 100% equal result. In some
cases the new default style does not require further tweaks, and in some
others styling as before is not possible.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>