Add Easy Language summary

Closes #67 (closed)

👉 Review here

Remains to be done:

• fix alignment with search and nav button
• check is ok to link only in the page header (i.e. not in the nav menu itself)
• check it is actually easy language? how?
• add a german version once the english one is fine (or separate MR)
• fix base_url so that it works also in review env
• fix breadcrumbs with "De" on the german version