Remove DocBook manual

.gitignore

doc/manual/advanced-topics/advanced-topics.xml

doc/manual/advanced-topics/cores-vs-jobs.xml

doc/manual/advanced-topics/diff-hook.xml

doc/manual/advanced-topics/distributed-builds.xml

doc/manual/advanced-topics/post-build-hook.xml

doc/manual/command-ref/command-ref.xml

doc/manual/command-ref/conf-file.xml

doc/manual/command-ref/env-common.xml

doc/manual/command-ref/files.xml

doc/manual/command-ref/main-commands.xml

doc/manual/command-ref/nix-build.xml

doc/manual/command-ref/nix-channel.xml

doc/manual/command-ref/nix-collect-garbage.xml

doc/manual/command-ref/nix-copy-closure.xml

doc/manual/command-ref/nix-daemon.xml

doc/manual/command-ref/nix-env.xml

doc/manual/command-ref/nix-hash.xml

doc/manual/command-ref/nix-instantiate.xml

doc/manual/command-ref/nix-prefetch-url.xml