There are basically two models of doc production (simplifying bit here) - distributed and non-distributed. Non-distributed is the passion model - you're making a doc because the subject matter is important for you to share. Often that means you've concepted it, shot it, directed it, edited it, mixed it, etc...and funded it (or fundraised it). Essentially you're working spec to shoot and finish it with the hopes it "goes somewhere."
Distributed means someone wants a doc about a specific subject and has a budget to produce that doc. It may find a life different at the end - but from the get-go there's a client with and idea of what it is and who the audience is. That can mean theatrical distribution or TV/streaming. In general, because there is a budget and client, this model has more complete crewing from research to field to post. If your goal is to work as an editor in doc, then you're basically looking at this style of production and you can focus your efforts on finding companies that specialize in doc/reality. Yes, some spec docs do hire editors but I wouldn't plan on making a career of editing spec docs.
Making a passion doc is absolutely a great thing to do, BUT unless you hit it out of the park it's probably not going to lead you to job after job doing what you're looking to do. You'll certainly learn a lot. That said, there is so much to learn by working FOR as many people as possible and paying attention to the ways they deal with various situations that may have nothing to do with technical skills. In the US, in terms of getting a relatively intro understanding of the top-to-bottom of doc production the best position to aim for is AP (associate producer). AP's in doc here tend to have a hand in every pot from the research phase to the field shoots to the post process. Most doc AP's I know develop very tight relationships with their EP's, Producers, DP's and Editors and, at times, will be trusted to do elements of all those roles. Especially once footage comes back - often AP's will help the producers string-out sequences for the editors.
Editors aren't often on-set outside of the spec passion projects where they're producer/director/dp/audio/editor / / / / . They rely heavily on the notes from the field, transcriptions, and those string outs that the producers assembled in order to speed-up the process. If your goal is to work into offline editing, it's super important to pay attention to the process as it passes from stage to stage. Taking a wholistic approach, seeing how elements are acquired in the field, how the producers tell the story, how the sequences are built - and finished by the editors - helps understand what does and doesn't work. Even seeing a talented editor take a string out and turn it into a segment is important in learning the nuance of editing which is a lot more than simply knowing some software.
AP isn't an entry-level position. But often doc companies are loyal to their people and will promote PA's that show good work ethic and attitude relatively quickly to AP positions. It's important to communicate your goals to the people you work for so they can help you get there.
Best approach is to work for people WHILE making your own doc. You get to try everything and apply lessons learned.