Anyone can write docs though. By making this post you’ve proven you’ve the skills for that. 
Depends on how you want to contribute. If you want to write code, look through the issues and pick one that seems easy, or create an issue to change something that’s been troublesome for you that you think you can fix. Some of the repos mark issues as good candidates for first time contributors.
The same goes for the docs with the addition that if you see something minor like a typo, there’s a link at the bottom of the page that takes you straight to where you can suggest a fix.
Or you can help users here on the forum by posting tutorials, answering questions, posting rule templates or widgets or blockly libraries. The barrier to entry here is very low.
Or, as discussed above, get into marketing and show off what you’ve accomplished, participate on other forums, create videos, etc.
But one very important thing to understand is you don’t need to be an expert in any of this to get started. You can learn as you go. We’ll help where we can. Many add-on developers had their add-on be the first Java anything they ever worked on.
There are two sides to this. We don’t have any way to manage a group of back benchers. We can’t say “Hey, Mark, X needs to be done and you have X skills. Get to it!” It’s up to you as the volunteer and the expert on what your skills are and what you are willing to work on to speak up and dive in. You need to find something you want to contribute.
Once you identify it, create a forum posting and an issue on GitHub where it can be discussed, especially if it’s going to be a lot of work. If it’s controversial, there might be discussion, compromises might need to be made, or it might be rejected because of some impact on something else.
Except for the creation of the GitHub account, all the rest of that gets handled when you click on that link at the bottom of the page.
Which PRs have received no feedback? In openhab-docs there are only ten open PRs, the oldest of which is from July 27th of this year. You will find few repos with a better response to PRs than that.