How to know what content to write about
Of course, your first instinct is to write content on how to use your product, as well as answering your FAQ's in a structured and easily searchable way.
But then what?
To know where the gaps are in your content, make use of the reporting tools in your elevio account to see what content people are searching for.
Empty search results
The obvious and quick wins are in the report that shows you what people are searching for that are yielding no search results. Make it a habit to check this at least weekly and make sure that you're covering all your bases (setting up a weekly reminder will help with this).
Study your analytics
Another good habit to get into is simply studying your analytics, to look at what articles people are reading the most. This can provide insights such as what areas of your site your users just aren't understanding well. Get into a groove of continually tweaking and improving those articles to make sure they're always relevant and teaching what you want to teach. These articles can also be a good gateway to educating users about other related content, so change it up and see if you can get some flow through learning happening.
Handle your negative feedback
Don't get disheartened when people leave negative feedback on an article. Often when people come looking for help, they're in a grumpy mood since something isn't working how they expected, or they're having trouble understanding something so they can tend to take it out on a faceless form.
But this creates an opportunity for you. People are providing you with their unfiltered feedback, this is worth its weight in gold. Use this all as constructive criticism to teach you where improvements can be made to your product, as well as how you can better improve your documentation so this kind of feedback doesn't come through again.