Put all build artifacts in a single directory

This PR suggests generating the examples' json and combined.js files in `build/examples/` instead of `examples/`.

As discussed previously, an during yesterday's dev meeting, it makes sense to place every generated file in the `build` dir.

More things should go into the `build` dir, for example `examples-list.xml` and `examples-list.js`. A separate PR will be created for the rest if people agree with the principle.