The easiest I imagine to install and maintain would be Wordpress. Then apply a knowledge base theme like the one from Hero Themes.

You can set up Wordpress using the one click install option here on Digital Ocean. This guide below outlines the process. Wordpress is open source (Free), the theme will cost you $149, but it’s relatively cheap and right on the money for what you have described as requirements.

Wordpress One Click Install

If you want the site to be editable by users such as StackExchange, I would look at something like discourse, which is modern day forum. If I remember correctly, I think the founder came from StackExchange. Once again you can do a one click install here on Digital Ocean. Guide link below.

Discourse One Click Install

Hope that helps.

https://wappalyzer.com/