Welcome!
I’ve wondered the same thing! It would be great if they could be contributed to.
I asked the JUCE team when I was new to the framework and recently again about contributing to docs. My understanding is that they don’t currently have the bandwidth to prioritize improving docs generally, as they are heads down on higher value items (the framework’s surface area is monstrous). But they will fix big things that are obviously broken.
They recently setup a CLA to technically allow community contributions to the main repo, which is a nice step. I opened a docs PR that fixes a few smaller problems to test the waters, but there’s not a functioning review process at the moment (that I know of).
Ideally, the community could take on some of the review burden, especially for docs/tutorials — which are particularly high value for people onboarding to the framework. I’m not sure if the JUCE team actually wants this, though, and it would take work to coordinate (bit of a Catch-22).