Add engineering page to handbook

Adding this, points out the need for some clarity on what is needed (if anything) for the various groups within Engineering.

The structure page lists CI, and developers not in the other groups as separated out, but it was not clear to me if indeed the distinction should be made in the handbook between backend and CI / backend and other developers. Looking for your input there.

The biggest gaping hole is that there is no page for backend (because all of gitlab-ce, gitlab-ee, etc. function in stead... but is there is probably value in separating out how development is organized from the gitlab-ce issue tracker itself?) and no page for frontend ...

\cc @sytses