Support building with no/less docs and use it more in CI

The initial trigger is just that our doxygen documentation isn't reproducible yet. This can't really be fixed from our side (apart from fixing all FIXMEs), but we can provide a sort-of workaround by disabling doxygen usage. We could use the nodoc build profile for that, but this actually disables a lot more documentation and even entire binary packages (now that it is implemented), so we also provide an apt-specific build profile to just disable the doxygen run.

Just adding options is boring through, so our Gitlab CI configuration is changed to stop building the entire documentation two times for no reason – which in theory should help the runtime, but in practice it is all over the place, but it feels slightly faster…

The biggest remaining puzzle is why amd64-root is slower than i386-user, even through the later is a complete build will root avoids building docs… so something seems broken with the ccache setup or something, but so it was before.

Similar attempts to improve runtime are made for our autopkgtests although this is far less about docs and more about using the libapt package under test for linking rather than building it from source. I would really like to not install all of our build-depends here, but that is also left for another day to figure out.