Migrate PDF translations to MDX pages

[maiertech]

In #494 a quick try to generate real pages with translations instead of linking PDF files failed. In this issue we need to look at how to handle i18n with Gatsby.

The goal of this issue is not to create a fully localized site, but to put the infrastructure in place to handle translations of selected MDX files instead of relying on PDFs.

There are 2 candidate plugins:

• gatsby-theme-i18n: Gatsby's official i18n theme. The theme has not made much progress recently. We need to clarify if they will continue with this or abandon it.
• gatsby-plugin-i18n: Community plugin. Status also not clear. Not sure if we want to rely on a community plugin.

Waiting for feedback on both plugins.