Marketing site: removed external CMS dependency, content now lives in code

The marketing site used to fetch content from an external CMS at build time. That added a network dependency to the build, an extra service to keep alive, and a separate review workflow for content changes.

Content now lives next to the code in a typed content layer. Translations ship through the same review process as everything else, the marketing site has one fewer external dependency, and local development works without spinning up a CMS first.

This is a developer-experience win that ends up being a user-experience win, because content updates now ship faster.