Allow new xhtml files to start at the right page

[Lululalu]

When viewing the content on a large screen, in two-page mode, new chapters, and sections, always start on the left page. This is bad because it adds plenty of unnecessary white space, specially in the frontmatter, where the content may be quite short.

To honest this is how most epub readers behave this way, Apple Books being the only exception that I know of that gets this right.

Additionally, as far as know, the epub specs allows to define the left or right page at the spine, and although it’s more of fixed layout thing, I’ve seen it used in reflowable as well.