Harmonize docbook build experience

[jtojnar]

While DocBook is pretty great for a software product, our documentation workflow is not. One of the issues is the ease of building the docs – normally the user would just open a nix-shell in the relevant directory and then call make to build the HTML output, this is not the case everywhere.

It is not sufficient to provide commands like nix-build nixos/release.nix -A manual.x86_64-linux in a README, since it is not fine-grained enough, slow and hard to remember.

Currently, we have a somewhat working Makefile in doc (Nixpkgs docs) but the one in nixos/doc/manual only contains a small number of targets and none for producing HTML. We should come up with a single set of targets that would be available in every Nix-related docs.

[jtojnar]

Still something I would like to tackle when I find time.

[jtojnar]

We are moving away from DocBook (NixOS/rfcs#72) so it no longer makes sense to invest time into this.