Hi all, thanks for the responses! 
@jarancio: Thanks! I completely forgot about shared Zotero libraries, even though I use Zotero for myself all the time! 
I suppose we can create a shared library there as an “index” to keep track of the disparate pieces of knowledge that builds up all over the place. IIRC Zotero has types for forum threads, multimedia resources, software, reports, etc. in addition to the usual suspects like academic papers. I am also a fan of Low-Tech Magazine.
I echo @Markos’s sentiment of the digital dark age. I feel like recorded knowledge has actually become more ephermeral now, because in the past if things are on paper - like books - they can last a loooong time but now not so much… This is why I love the Internet Archive. In fact, we could archive some GOSH outputs there, too. I vaguely remember @hikinghack saying something about storing things there but I might be wrong.
@marinappdf: Thanks for linking to that paper, it articulates so well many of my worries that I haven’t been able to focus and communicate. It’s very true that the problem isn’t just about which tools we use to organise and archive our collective knowledge, but also that we need people to do the task. I don’t have a good solution in mind, but that’s why I wanted to at least begin a reflection.
I think wikis were discussed in the other thread about self-hosted/open source apps for GOSH, and the problem we identified was that it takes a lot of effort and dedicated people to maintain/organise…
@solstag: Good point about defining what is “knowledge”. I think what I’m trying to get at is that there is a lot of informal “know-how” that comes from the conversations in GOSH, such as discussions in this forum or elsewhere. We learn a lot from this but it’s easy for this kind of knowledge to get lost. For example (and I know this is not an open science hardware-specific example), we had a long forum thread about election voting methods. We collectively learned something from that discussion, but eventually that thread will be buried deep in this forum. Most of us would have forgotten where it is, and eventually no one will even remember that the discussion took place or that we’ve investigated and learned about the different solutions. In other words, even if a forum thread or GitLab/GitHub issue technically exists, after enough time no one remembers that it’s there or what we learned from it. Basically, that knowledge is effectively “lost”. Having good organisation and archiving of more “formal” knowledge, such as papers or the publications on the GOSH website are already hard, but for informal, ad-hoc knowledge production it is even more difficult!
My summary
From what I can tell, looks like:
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A shared Zotero library (I think they’re called Zotero Groups?) might serve as an index to track various “pieces” of knowledge that are all over the place.
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But in addition to having good tools, a bigger challenge might be having the people to collect, organise, and archive that knowledge.
I suppose something we could try is to simply start experimenting with different solution. Try starting a Zotero group/library to track our knowledge and see how hard it is. Maybe play around with a wiki. And once we’ve organised something, we could try to make a separate archive with Kiwis as @jmwright suggested. I don’t really know… 
I am also really curious how other organisations have tried (and failed?) to track their learnings/knowledge???