The user guide is still being updated for 0.6, I know meme is hoping to push some updates very soon on that.
If you want to insert a special marker in the html to then later do a split all at once, use Insert->Split Marker and then Edit->Split At Markers. There are keyboard shortcuts for both defaulting to Ctrl+Shift+Enter and F6 respectively. It can be useful to do a find/replace to insert these as well, such as to insert before chapter headings.
If you want to split the html immediately, then use Edit->Split At Cursor (Ctrl+Enter) for which there is a toolbar button for it.
CSS page-break-before instructions have no meaning to Sigil - they are only of interest to the ereader device/software.