Documentation for `ref` and `relref`.

- Rejigged the weight of the extras/ content for the new crossreferences
  page.
- Used the new {{</*…*/>}} format for documenting highlighting and to
  prevent a warning about the missing `fig` shortcode.
Added AuthorList, Author, AuthorSocial, SiteSocial, Image and Video structs
Added Page.Author(s) functions
Added schema, opengraph, twitter_cards, google_news metadata templates
Added "" template
Added delimit & sort template functions, tests and docs
Fix unicode menu URLs

Menu urls like /categories/новости-проекта would turn into /categories/d0bdd0bed0b2d0bed181d182d0b8-d0bfd180d0bed0b5d0bad182d0b0, which is illegal, while the directory under the categories/ is created with the original name. It results in 404 not found error.

This commit fixes that by make sure that SanitizeUrl() is called last.

Fixes #719