doc: overide the default margin for "ul.simple > li"
by default, the user agent's css sets `margin-block-end` of "ul ul" to
"0", which renders the unordered lists in release notes cluttered. as we
are using nested unordered list to present the changes in different
components.
in this change a customized css is added to reset the begin and end
margin of ul to 1em, which is the default value of top level ul.