I’m not sure I have the right answer, or if there even is one, but with complex topics like this one I’ve found it reduces the support burden to come at the problem from two angles.
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Having the info in the docs in the places it should be in the hope that most people will see it and understand
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For particularly contentious/confusing topics that constantly come up - have an extra and dedicated in depth page.
Yes it may be redundant but it saves time overall for support ‘staff’ when a further reduction in those that are confused can be achieved with a simple ‘read this page for further in depth detail’.
Also a link along the lines of ‘see this page for in depth info on this topic’ can be added to the other sections which would solve the "how would they know they need to read a page called “Using _index.md?”.
What’s for certain is that the current docs discoverability on this topic isn’t working (so an updated/rework/greater visibility/rewrite would probably be a good idea anyway). I think the current top two posts here on the are regarding confusion over it. This one in particular:
Hopefully things like that thread will die down as we get further away from pre “everything is a page” but for now at least possibly having a dedicated page with info worked into the ‘right’ ares of the new docs could help.