upgrade to docify v0.2.4 (#1653)

Upgrades to docify v0.2.4 which adds support for exporting trait and
impl items in addition to regular items. This fixes @liamaharon's
https://github.com/sam0x17/docify/issues/9 issue.

See the release notes for more information:
https://github.com/sam0x17/docify/releases/tag/v0.2.4