content: fix and update build process (part 1)

The grandiloquent title does not match this specific MR. For now, I propose commits that are "just" crucial changes to have a functional build, which was not possible until now, at least not on Windows.

The build pages in general have a lot of small problems, but as they aren't urgent, being more subjective, I decided to leave it to another MR: !40 (merged).