[Doc] Canonical URLs metadata #914

[Harvey1214]

No description provided.

[hakenr]

We need this to cover most pages (if not all). See the list of duplicates identified by Google Search Console.
Consider using a root cascading value to manage the registrations. I think the first one should always take precedence (no need for MainComponent). We can add a special <PageCanonicalUrl /> component to act as the initial registrant in cases where the order of components on the page doesn't satisfy the rule.

Duplicate URLs without canonical.txt

[hakenr]

see my previous comment

[hakenr]

consolidated, merged master, lgtm