Function site_version_segment 

pub fn site_version_segment(tail: &str) -> Option<(&str, &str)>

Splits the tail of a SITE_ROOT match into its version segment and the rest.

None when there is no / to split on — …/atune/ at the end of a sentence is the site’s own address rather than a page citation, and a bare …/atune/concepts with no trailing slash cannot be told from a version segment named concepts; both are left to the caller to ignore.