Add link rel=canonical to default themes

[preaction]

For content that is available in multiple places, via syndication or some other means, good search engine practices require us to decide which one is canonical. Otherwise, each individual instance of the content will split the ranking, taking their own portion of the relevance that might otherwise be applied to a single instance of the content.

Each document should allow the "canonical" link relation, and each layout in each theme should show the canonical link.

• [ ] Add the <link rel="canonical" href="..."/> to all default theme layouts (share/theme/*/layout/*)
• [ ] Add a test to t/theme/check.t to show the canonical link tag appears when needed, and does not appear when not needed
• [ ] Document the "canonical" link relation in the Document class (lib/Statocles/Document.pm)
• [ ] Document the "canonical" link relation in the new content guide (site/docs/content/index.markdown).