Hi Felix,

This is an example set up, how the wiki could work and also how external user could make contributions.

TOC page, categories  to create hierarchy

Table of Contents
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For Editors:

Lost your password?

All categories
Category (filtered)

Used Plugins:

YadaWiki

Also,I have used redirection for the /wiki > /welcome-to ( since the headpage (i.e. /wiki is not editable) to work around wiki-landing pages  and  menu localization.

Advanced Access Manager plugin

users will need a login account. With this account they will only have access to edit wikipages(WikiAuthor)