This is the one we used. https://en-gb.wordpress.org/plugins/wp-knowledgebase/ As you can see, you can organize articles in categories and there is even a search box. Initial feedback is good so far. A knowledgebase system is essential in all companies regardless of the feedback some users might give (usually those who are bored to search for answers), since all knowledge should be stored centrally. If a key person leaves, all his/her knowledge will be there!