My advice would be to look at the documentation for projects that you use or like. The state of documentation in open source projects overall is not great, so probably you can find a project that could use some help. It might take a while to develop a good working relationship with the developers, but once they realize you're a good writer and serious about helping hopefully they'll give you enthusiastic support.
I have had some luck with suggesting better wording for projects submitted via Show HN. Some of that has been adopted and I've been sincerely thanked a few times. I've documented a little of that as part of a kind of portfolio.
I haven't tried super hard to pursue this. If you want to get involved in earnest, you need a little luck (plus some quiet, patient persistence). You should look for situations with an extensive need for writing help plus good personal rapport between you and one or more key people.
That's basically how you get involved in something when they aren't really looking for you. You identify a need you can meet, then try to establish those personal connections that aren't overly personal, but are personable enough where you can have good and useful conversations about stuff not currently on their radar without them giving you the cold shoulder because they see you as a waste of time.
I think that's the biggest thing I wrestle with is convincing people I have something of value to offer without sounding like "You suck!" It's really tricky when it's knowledge work.
The other thing about writing technical documentation is you have to understand the tech some to talk about it. I occasionally do resume work and I mostly do that for experienced tech people who probably know me through HN in most cases. I can edit their resumes even though I'm not a serious programmer because I have some idea what the hell they do for a living.
This is a really big thing for the tech world. Communicating well means you first need to understand the topic and the tech world routinely beats its head against a wall because managers and salespeople and so forth don't actually understand the tech well enough to be genuinely effective. It leads to lots of "They did this stupid thing because they honestly have no idea what the tech actually does."
This seems to be a factor in some of the debacles where some startup company has stuck "AI" in the name, absolutely can't deliver on that promise and they are getting sued because of it.
A lot of "tech" solutions are like that scene in Men in Black 2 where they open the mail sorting machine and inside is a guy with many arms flinging mail as fast as he can.
Many non-technical people honestly don't know the difference between a box of people and an actual machine-based solution. If you are crowd sourcing work from many remote workers and getting results on a website, they just see the website and go "Well, neat! Computers are amazing!"
So be prepared for writing technical documentation to be hard at first and involve a lot of research. But if you get good at it, you might find that people will seek you out because a lot of writers simply aren't qualified to communicate on the subject. They don't have the background knowledge.
Documentation is one of the most important things for the well-being of a project. Your contributions would be extremely valuable. So if you think what we do is worthwhile, please get in touch.
Full disclosure: I am the maintainer
So don't short-sell yourself. :)