A trick I've used effectively is to have a "doc of docs" - a document that tells you where all the other documents for a project or team live.

You can do this as a wiki page or a Google Doc. The important thing is that the answer to the question "where's the documentation for X" should ALWAYS be "it's in the doc-of-docs".

Then you can take it a step further: you can say "it's in the doc-of-docs... and if it isn't, when you find it, add it to the doc-of-docs!"

It's a universal truth that documentation for projects and teams ends up scattered in many different places. A doc-of-docs is a lightweight technique that can really help here.