- your centralised wiki lists all the repositories and explains what they’re for;

- each repository has a detailed commit history, allowing you to understand the thinking behind each line of code.

Edit to add: you can search across multiple repos with Github’s “search in organization” feature, and it actually works very well. I usually find that much more useful than searching through a wiki that’s separate from the code (although maybe I just haven’t ever seen a wiki that was sufficiently well organized?)