Add canonical URL for all routes

[cnrdh]

akvaplan_fresh needs canonical URL / sitemap support for all routes

With Deno KV it's now extremely simple to store the delivered content of all canonical pages (if this can be grabbed by middleware?)

A KV site search index could run as a separate Deno Deploy Search API, updated when a page changes

KV store

• ["canonical", lang, pathname] – the unique text content of a individual page

Probably use https://github.com/nextapps-de/flexsearch – the index would have to created on each isolate start since it's probably bigger than Deno KV document size limit

[cnrdh]

Search is implemented using orama – without a sitemap and without canonical routes.

Open Graph also supports alternative language routes, support for the 4 required properties added in https://github.com/akvaplan-niva/akvaplan_fresh/pull/398/files