Just picking up random work seldomly results in good quality code. Picking a task you want to do (either one in our tickets, or one you invent) makes it a lot easier to achieve a result.
As such, I don't see it as a problem when you just start creating your mod. If you need changes to the engine, we can still guide you, and then incorporate the changes if they are good. We could also incorporate some of the things you add as a mod, if they're coded decently.
You'll see that all art and most gameplay code is implemented as a mod (the public mod), so if you want to create your own game using the engine, you can just replace that mod with your own mod. And either distribute it as a complete game (engine + mod), or distribute it as a mod people have to install in their 0 A.D. installation (note that people can disable the public mod, so total conversion mods are possible).
I do have to warn you for the legal aspects though. If your art gets mixed with existing game art (including the GUI art), you should also distribute your art under the CC-BY-SA license. If your code gets mixed with code from us, you should also release your code under GPLv2+.
About the manual, that's mostly updated, some pieces may be missing, but there shouldn't be a lot of wrong parts. If you find something wrong, just drop by on #0ad-dev on Quakenet to discuss it, and it can be fixed.