It's an interesting idea. I don't know what the market would be for the services you're describing, but it's certainly a way to distinguish yourself from more traditional programmer/writers.

Traditional documentation jobs involve creating references and conceptual content describing a project and there are a number of different tools you could use. Also, there are opportunities for so-called "glue" languages to help build documentation experiences. (I've used Perl, node.js, java, etc.)

Perhaps you could put together a demo site showing the type of work you'd like to perform and the type of results you'd provide. (I've found a GitHub pages to be a relatively pain-free way to deploy such sites.)

You might also consider signing up for sites like freelance.com, upwork.com, and so on. If you're also considering more traditional tech writer contracts, you might consider communities like https://www.writethedocs.org/, which runs a very active Slack channel, hosts job postings, and so on.

Good luck!

--f