devkitPro

I assume the answer is no (I have not found anything online), but I ask anyway:

Is it possible to create a built-in user manual like in commercial titles?

I am referring to the "Manual" button that appears at the bottom left when a commercial game Icon is selected in the home menu.

It opens an inline-help system.

We don't support installation of homebrew. This reduces functionality like argv support as well as being slow and clunky to install. The tools required to create installable archives are dubious and, of course, installed homebrew is technologically indistinguishable from pirated content which can lead to users consoles being blocked from Nintendo's servers.

So no, and ideally only distribute your homebrew as .3dsx.