Marketing pages, blog and changelog

The public site grows its last static surfaces before the launch post.

Adds the marketing pages /, /trust and /about, in English and German, with security.txt renewed by every deploy.
Adds /blog and /changelog as language pairs, each with an RSS feed per language.
Adds link-preview cards for Posts and Entries, drawn from the same titles the pages show.

This footer is one of four places a joke is allowed. Tigby is holding it for a good one.