For me, I browse through the GitHub UI and I also keep local copies of Zig (for various releases) so I can `git grep` and glance through lib/std/mem.zig or lib/std/array_list.zig or (as in this post) lib/std/json/static.zig.
I'm not saying it works for everyone but frankly, like motiejus it sounds, I don't mind the minor challenge of being forced to read source code.
Docs is something they'll probably tackle before version 1.0 after the language is stabilized and they clean up the standard library. Not much sense in writing docs if they are going to need rewriting soon.
Right now the best doc is to look at the tests in the code itself.