[Documentation] mark and archive outdated pages

[chseifert]

Some wiki pages are already marked as outdated. We need to go over them again and see what is still up-to-date. This is an issue that will be assigned to every WP leader to check the pages s/he is reponsible for.

[ThomasCerq]

Done for WP6, except for this page: https://github.com/EEXCESS/eexcess/wiki/%5Bcurrent%5D--Request-and-Response-format-for-call-to-federated-recommender-and-privacy-proxy
The content is obsolete (and can be deleted) wrt WP6, but I don't know if it's still needed by other WPs...